1 Introduction

Aristotle approaches the study of perishable life via separate studies of plants and animals.Footnote ¹ He deals with certain aspects of perishable life in common for animals and plants in the short essays collectively known as Parva naturalia. But it is far from obvious that dealing with those aspects amounts to a full-fledged study of the phenomenon of perishable life. What Aristotle says in common for animals and plants is in fact embedded in his study of animals.Footnote ² Rather than overcoming the division of the study of perishable life into separate investigations of animals and plants, this idiosyncratic strategy ends up strengthening it. In the end, the reader of the Parva naturalia comes away with the distinct impression that the study of animals remains Aristotle’s primary, and indeed overriding, concern. My task in this chapter is to look at how Aristotle approaches the topic of animals with a focus on the methodological decisions that control and indeed shape it.

The first thing to note is that the writings concerned with animals comprise over 25 percent of the surviving Aristotelian corpus.Footnote ³ If we concentrate on the extant works on natural philosophy, this number becomes truly impressive: approximately 60 percent of the extant works on natural philosophy are concerned with the study of animals.Footnote ⁴ This number is even more staggering when we consider that the writings on natural philosophy are over 40 percent of the extant corpus of writings transmitted under the name of Aristotle. Clearly, Aristotle aimed at offering a scientific account of the natural world in which animal life was by far the most conspicuous explanandum.

We cannot say with confidence when or where Aristotle wrote these works. For one thing, we know too little about Aristotle’s life. For another, we have no independent way to fix the relative chronology of his writings. The first period away from Athens is often regarded as especially important for the development of Aristotle’s science because the zoological data collected in the HA appear to refer to places on the coast of Asia Minor. Scholars have used this observation to establish a connection between the research activities that constituted the basis of the impressive corpus of writings on animals and the years that Aristotle spent away from Athens – especially the years of residence in the Troads (Assos) and on the island of Lesbos (Mytilene).Footnote ⁵ While suggestive, this connection fails to do justice to the centrality of the study of animals to several aspects of Aristotle’s philosophy. It also presupposes that the works on animals that have reached us are a finished product that can be ascribed to a particular period of Aristotle’s philosophical development. Scholars have often assumed that Aristotle completed his study of animals and then turned to different endeavors. But this is far from being an obvious, let alone safe, assumption given how little we know about Aristotle. In connection with this last observation, it is worth recalling the conclusion that James G. Lennox has reached in an article explicitly concerned with the question of the place of the study of animals in Aristotle’s intellectual biography:

For Lennox (and Balme), the writings that have come down to us are best understood as a work in progress. In the rest of this chapter, I would like to give the reader at least an idea of how Aristotle conceives of his task in these writings.

2 The Collection and Presentation of the Zoological Data

Posterior Analytics II is concerned with scientific inquiry (ζήτησις).Footnote ⁷ At the outset of the book, Aristotle famously states that the things we seek are the same as the things we know scientifically.Footnote ⁸ They are four in number:

1. (1) the fact that (τὸ ὅτι)

2. (2) the reason why (τὸ διότι)

3. (3) whether something is (εἰ ἔστι)

4. (4) what something is (τί ἐστιν).

It is possible to know the fact without knowing the reason why. In a similar way, we may know whether something exists without knowing what it is. But whenever we know the fact, we seek the reason why. In a similar way, whenever we know whether something exists, we seek what it is. In other words, there is a natural progression in the scientific inquiry from (1) to (2) and from (3) to (4). While a scientific inquiry directed at (2) aims at an explanation, one geared toward (4) has definition as its goal. For Aristotle, explanation and definition are not two disconnected goals. On the contrary, the search for the explanation and the search for the definition converge toward one and the same result. Consider the following passage:

I will not deal with the question of how definition and explanation go together in Aristotle’s theory of science. What is immediately relevant to the present discussion is that the scientific enterprise as is understood by Aristotle advances in stages. At the most general level, science proceeds from a stage in which the existence of the things that are to be studied and the facts about them that need an explanation are established to a stage in which those things are defined and the relevant facts about them are explained.Footnote ¹⁰ The theory outlined so far is general; as such, it applies to all scientific domains.Footnote ¹¹ The study of animals is no exception to the rule: a zoological investigation aimed at establishing the facts about animals in need of causal explanation should precede the inquiry into their causes. What Aristotle says about stages of inquiry in his Posterior Analytics does not require him to present the facts and their explanations separately in distinct works. In fact, the division of the study of animals into treatises that are concerned with offering causal explanations on the one hand, and a treatise devoted to presenting the facts in need of an explanation on the other, is a unicum in the Aristotelian corpus.Footnote ¹²

Following Lennox, we may want to distinguish between the following three theses:

1. (1) A zoological investigation that establishes the facts in need of explanation should precede the inquiry into their causes.

2. (2) The writing of a treatise presenting the zoological facts in need of explanation should precede the writing of treatises concerned with their causal explanations.

3. (3) The study of a treatise presenting the zoological facts in need of explanation should precede the study of treatises concerned with their causal explanations.Footnote ¹³

While the general theory of inquiry outlined in the Posterior Analytics commits Aristotle to (1) and (3), this theory tells us nothing about (2). Any scientific enterprise should begin by collecting the relevant data, which will then become the object of an investigation that has the explicit goal of explaining them – thesis (1). Moreover, the treatise that collects and presents those data should come before any one of the treatises that explain them in the order of study – thesis (3). But we should refrain from drawing any chronological conclusions as to the relative dating of these writings – thesis (2). The study of animals is no exception to the rule. In fact, this study illustrates this complex situation in an especially clear way.

The Greek manuscript tradition has transmitted a treatise entirely devoted to the collection and presentation of the zoological data – namely, HA. This work comes before the writings concerned with the explanation of those data in the order of exposition. But it does not follow from this observation that HA was written before those writings. A lifelong engagement with the zoological writings led David Balme to formulate a developmental hypothesis, according to which HA is the last of the works on animals written by Aristotle.Footnote ¹⁴ This hypothesis, known as the Balme hypothesis, entails that Aristotle continued to revise the main results reached in his study of animals until the very end of his life.Footnote ¹⁵ This hypothesis has not gained universal acceptance.Footnote ¹⁶ For our purposes we can bracket the Balme hypothesis and concentrate on the aims of HA. These aims have been greatly clarified by the work of Balme. Against the prevalent (at the time) view that HA is primarily concerned with offering an exhaustive, hierarchic classification of animals, Balme argued that this treatise is “a collection of all observable differentiae, collected qua differentiae.”Footnote ¹⁷ “Differentia” (plural “differentiae”) is the Latin translation of the Greek “διαφορά.” By this term, Aristotle means “any way in which X may differ from Y.”Footnote ¹⁸ A possible rendering in English is “difference.” According to Balme, HA is primarily concerned with animal differences – namely, the ways in which animal kinds differ from one another; for instance, the ways in which they differ with respect to their modes of generation or their modes of locomotion. On this reading, a first goal of HA is definitional: arriving at a definition of the relevant phenomena via the collection and analysis of all the relevant differences. None of these differences are by themselves sufficient to reach an adequate definition of an animal. But if we divide the different kinds of animals by adopting the method of division by multiple differences Aristotle outlines in PA I 2–3, then we can reach the relevant definitions.Footnote ¹⁹ Another, related goal of HA is explanatory. Aristotle refers to this second goal in a key methodological passage where he tells us that “after this [collection of all the relevant zoological data] we must try to find their causes.”Footnote ²⁰

This picture of the aims of HA has been considerably refined by Allan Gotthelf and James G. Lennox.Footnote ²¹ They have shown that Aristotle is concerned not only with laying out animal differences but also with finding out significant groupings of animal differences. For instance, Aristotle is often interested in what a large kind (e.g., birds) has in common with other large kinds, and only then discussing what distinguishes birds or is even unique to them. The same phenomenon can be observed when Aristotle is concerned with sub-kinds of animals (e.g., kinds of birds). Again, the focus is on what a certain sub-kind shares with other sub-kinds followed by a discussion of what is unique about that sub-kind. The theoretical motivation for this idiosyncratic procedure can be traced back to the requirement that explanations be given at the right level of generality.Footnote ²² This requirement shapes Aristotle’s overall research project beyond the study of animals.Footnote ²³ Hence, we should not be surprised to see it at work also in HA. Here Aristotle not only collects the relevant differences but also lays them out with a view to generate explanations that reflect the methodological procedures outlined in his Posterior Analytics.

What emerges from this picture is a study of animal differences that is geared toward not only definition but also explanation.Footnote ²⁴ Since explanation must conform to the procedures of scientific inquiry outlined in Aristotle’s Posterior Analytics, the relevant differences are not only collected but also presented with a view to the subsequent stage of inquiry. So HA is not completely innocent with respect to explanation to the extent that the relevant zoological data are not only collected but also organized for the sake of their subsequent explanation. In this sense, it is better to think of the ὅτι–stage of inquiry as a pre-explanatory (or pre-demonstrative) stage of investigation.Footnote ²⁵

I would like to end this introduction to HA by recalling a famous passage that marks the transition from the outline of the relevant animal differences (HA I 1–6) to the collection of the zoological data offered in the rest of work. This passage points forward to the goals of explanation and demonstration via a discovery of the relevant causes:

3 Pre-explanatory Strategies in HA

In this section I would like to look at the way in which Aristotle organizes and presents his zoological data with a special focus on the bodily parts of animals. Animals differ from one another not only by having (or lacking) certain parts but also by having them arranged in different ways. Aristotle’s systematic study of how animals differ with respect to their parts takes up most of the first four books of HA (HA I 7–IV 7). Aristotle tells us that his presentation of the zoological data begins with parts rather than modes of life and activities of animals because it is “chiefly, and firstly, with respect to the parts that the wholes also differ.”Footnote ²⁷ According to Aristotle, different animals have different body parts because they are engaged in different activities that are constitutive of different modes of life. For instance, fishes have fins because they are marine animals: they displace their bodies in water by means of their fins. We can restate this insight as follows: fins are useful to fishes for swimming, which is their characteristic activity (πράξις). The explanatory connection between a part such as fins in fishes and the activities that are constitutive of the different modes of life (βίοι) becomes a major concern at the explanatory stage of his inquiry. At the pre-explanatory stage, Aristotle’s main goal is to offer a survey of those parts with a view to their subsequent explanation. The question that concerns us is how this survey is organized and what are the epistemic principles controlling the presentation of the relevant zoological data.

The first thing to note is that Aristotle is quite explicit about his overall strategy in HA. His methodological explicitness is not only remarkable but also unmatched in antiquity. While Aristotle is not the first to engage in a description of the body parts of the human being and the other animals, he is unique in his self-conscious attempt to provide his reader with clear instructions on how the relevant data are collected and presented. Aristotle’s most important move is made in HA I 6, where he tells us that the review of the parts of animals begins with a discussion of the human body because the human being is the animal that is most familiar to us. Aristotle justifies this decision with the help of an example taken from our everyday life. We establish the value of some money in a foreign currency by translating it into our own currency:

Beginning the review of bodily parts in animals with the human being is introduced as a non-arbitrary decision – indeed, as a first, necessary step. Apparently, as investigators of the animal kingdom, we have no choice but to start our review from the human body because of our proximity to it. This proximity makes the human body the most familiar to us, and so the natural starting point for a systematic review of the body parts of all known animals. Aristotle begins his review of the differences in animals with the external parts, starting from the top of the human body and moving down. He then turns his attention to the internal parts of the human being. In a second key methodological passage, Aristotle tells us that we face special challenges when we look inside the human body. The internal parts, Aristotle says, are the least known to us.Footnote ²⁹ While Aristotle does not elaborate on the reasons for our relative lack of knowledge about the inside of the human body, we should keep in mind that the dissection of the human body, including its dissection post mortem, was forbidden during almost all Greek antiquity.Footnote ³⁰ As a result, ancient Greek knowledge about the internal parts of the human body was largely dependent on knowledge acquired from the dissection of animals. Aristotle’s survey of the internal parts of the human being betrays this epistemological dependence. For instance, Aristotle says that the stomach of the human being resembles that of a dog; he also maintains that the lower part of the intestinal tract of the human being is like that of a pig; finally, he claims that the human liver is like that of an ox.Footnote ³¹

The asymmetry between Aristotle’s knowledge of the internal and external parts of the human body is well known.Footnote ³² I am recalling this asymmetry here because it suggests that Aristotle’s decision to begin his review of the bodily parts of animals starting from the human being cannot be explained only in terms of our familiarity with the human body. As a matter of fact, this familiarity does not extend to include the inside of the human body. If, therefore, we want to understand why Aristotle reviews both the external and the internal parts of the human body before turning to the parts of the other animals, we must bring to the fore another methodological assumption that shapes his review of the body parts of animals in HA I–IV. This assumption is made explicit in the discussion of the relative position of the external parts in the human body.

In this passage Aristotle recalls his thesis that living bodies display up to three functional parts: up/down, front/back, and left/right. These functional parts are most clearly articulated in the human body because of its erect posture. Other animals either do not have all three dimensions or they have them confounded to a degree.Footnote ³⁴ Aristotle does not mean to say that these functional parts are not present in other animals; he only means to say that in these animals these functional parts are found together in the same place. Of course, we can tell them apart even when they are mixed up in the same place since we can trace them back to different powers of the soul present in the living body. For instance, the front is where the sense-organs are implanted, whereas the up is the entry point of nourishment. While this functional distinction may not be evident to sense-perception, the investigator can always see it by means of reason. Consider a snake: we can easily recognize a front and a back in its elongated body; however, we cannot distinguish, or cannot distinguish easily, an up and a down, or a right and a left, in that body. The reason is to be found in its elongated shape. And yet we possess a theory enabling us to argue for the existence of these bodily distinctions regardless of whether we see them. To the best of my knowledge, Aristotle is the first to equate the living body to an organized body, and he is also the first to distinguish the different kinds of perishable living beings in terms of the different levels of their bodily organization. It should also be clear that different degrees of organic unity correspond to the different levels of bodily organization. The highest level of organic unity is found in the living body that displays the greatest level of organic complexity.Footnote ³⁵

I will not engage in a full review of all the relevant passages where this conception of the perishable living body is at work.Footnote ³⁶ I am content to refer the reader to Aristotle’s De anima, where the soul is defined as the first actuality of a natural, organic body that has life potentially.Footnote ³⁷ Much ink has been spilled on the meaning of “organic.”Footnote ³⁸ To be sure, Aristotle means to say that the living body is an organ of the soul. In other words, the whole living body is an instrument that the soul employs in the exercise its powers.Footnote ³⁹ But this living body is to be minimally organized in a certain way if it must perform its function in an optimal way. The brief allusion to parts in plants that follows immediately after in the text may be meant to drive home this very point. When Aristotle says that roots are to plants what mouth is to animals, he reminds us not only of the analogous functions that these parts have in plants and animals but also of the existence of distinct levels of organization in the different kinds of perishable living bodies. So I do not think that the traditional reading is hopelessly misleading. According to this reading, which goes back at least to Alexander of Aphrodisias, the natural body that is potentially alive is organic in the sense that it consists of bodily organs.Footnote ⁴⁰

What matters for our present purposes is the highly abstract conceptualization of the perishable living body at work in HA. A certain idea of organization is invoked when we are told that the human body displays the relevant organization in the clearest possible way. This organization is clear to sense-perception only in the case of the human body, and only with reference to its external parts. What I would like to take away from this brief discussion is the following conclusion: Aristotle begins his review of the parts in animals from the human body not only because the human body is the most familiar to us but also because the human body can serve as an objective standard of reference. By offering a review of the parts of the human body, Aristotle launches into a study of what he takes to be the paradigmatic case of the perishable living body. Equipped with the conceptual resources developed in the context of this study, he can engage in the study of the living body of other animals. As Aristotle himself says in the last passage cited above, this strategy ensures that the investigators of the animal kingdom do not overlook anything of importance as they review the body parts of other animals.

Two epistemic principles control the review of the relevant zoological data. While the first requires us to proceed from the more knowable to us, the second mandates that we proceed from the more knowable per se (alternatively, more knowable by nature). These principles need not clash with one another. According to Aristotle, the human body not only happens to display the highest degree of organic unity but it also happens to be the most familiar to us. If this conclusion is accepted, the strategy Aristotle adopts for the review at the pre-explanatory stage of inquiry is close to the one he employs at the subsequent, explanatory stage. Let me return to how Aristotle negotiates the transition from the study of the uniform parts to the study of the non-uniform parts at the outset of PA II 10. Aristotle begins his discussion by stating that all animals must have a part by which they take in the nourishment (the mouth), and another part where the nourishment is processed (the stomach). There is, however, a great deal of variation in bodily organization beyond this important commonality. Aristotle recommends handling this variation starting from the human being:

In this passage Aristotle is more forthcoming about the reasons why we ought to start the study of the non-uniform body parts from those of the human being. While Aristotle still mentions our familiarity with the external parts of the human body, he now adds that the anatomical arrangement of the non-uniform parts in the human body is the most natural. This claim holds not only for the external but also for the internal parts. Finally, Aristotle say that the human body alone displays what he takes to be the most natural arrangement of body parts. When we take all these reasons into account, we have no choice but to start our review of the non-uniform parts from those of the human being.

What is exceptional about the human animal is its erect posture, which aligns the human body with the body of the universe. The human body is regarded as the body that displays the greatest level of organization because the human being partakes of living well (as opposed to merely living). To unpack this idea, we must return to the Aristotelian insight that the body is an instrument employed by the soul to discharge its powers.Footnote ⁴³ According to Aristotle, the human soul has the power to live in accordance with reason. The passage just quoted does not elaborate on the connection between displaying a greater level of bodily organization and rationality, but we may think of reason as a power that the human being alone has in addition to the other cognitive powers it shares with the nonhuman animals. It is also prima facie plausible that the presence of this additional (higher) power requires an additional (higher) level of bodily organization.

I have elaborated on the methodological remarks Aristotle offers in his review of the internal and external non-uniform parts of the human body to show that Aristotle employs the human animal as a starting point for his anatomical study of nonhuman animals. The implicit rule of inquiry at work is that our study ought to start from that which displays the greatest level of organization since this is also the thing that displays the higher level of unity. The clearest and most general formulation of this rule is found in a fragment from the lost Protrepticus.Footnote ⁴⁴ By the end of HA I, the review of the human body is complete. Aristotle turns to the parts of the other animals at the outset of HA II. His survey of these parts is over only at the end of HA IV 6. We should not underestimate how complex is the project of offering a systematic survey of all the parts of all the animals other than the human being. At the very least, the investigator is expected to have developed a strategy to present the zoological information in an orderly manner.

Two conceptual distinctions are at work in Aristotle’s survey. The first is the division of bodily parts into external and internal parts; the second is their division into uniform and non-uniform ones. When we combine them, we obtain a survey of the zoological data that begins with the external non-uniform parts, continues with the internal non-uniform parts, and ends with an examination of the uniform parts. But there is at least another key methodological insight that controls how Aristotle presents his zoological data. This is the rule of inquiry that requires us to proceed from those animals that display a more organized, or at least a more determinate, living body. Admittedly, Aristotle never invokes or states this rule; but as he moves away from the human body, he appears to find progressively less organization and less determination. We may object that Aristotle finds progressively less organization and less determination precisely because he is less familiar with these animals. This is certainly true, but we should not forget that these animals occupy a lower position in Aristotle’s scale of nature. In his view, they occupy a lower position because they are less complete or even less perfect. We are back to the methodological insight that informs the architectonic decision to begin the examination of the non-uniform parts of animals from the human body because this is the most organized and most articulate living body.

To fully appreciate how Aristotle moves on, and indeed forward, with his project of a systematic review of the bodily parts in animals, we need to return to a divisoin introduced in the previous chapter: that between blooded and bloodless animals. To understand what this division entails, we must return to the outline of animal differences advanced at the outset of HA, where we are told that all animals are supplied with moisture, the privation of which results in death.Footnote ⁴⁵ The moisture present in blooded animals is of course blood, which runs in the blood vessels. The presence of blood and blood vessels entails the presence of a few related parts. Among them, I recall the heart, viscera, and flesh. When these bodily parts are absent, we should posit the presence of a set of bodily parts that is functionally analogous. I will return to this important aspect of Aristotle’s theory in a moment. For the time being, let me stress that the review of blooded animals grows out quite naturally from the discussion of the bodily parts in the human being since the latter is the most perfect among the blooded animals. When we look more closely at how Aristotle reviews the zoological data about blooded animals, we discover that the following groups of animals are discussed (in this very order): live-bearing four-footed animals, egg-laying four-footed animals, birds, fishes, and snakes. In other words, the review of the zoological data is organized with the help of the large kinds (μέγιστα γένη) introduced in HA I 6.Footnote ⁴⁶

In some cases, these kinds are already identified by the Greek language (fishes, birds, snakes).Footnote ⁴⁷ When they are available, Aristotle is happy to adopt existing names. By adopting these names, Aristotle is also turning them into terms of art. But the absence of a name in a natural language does not prevent Aristotle from recognizing a new group when he has reasons to think that the animals in question possess a common nature. Clearly, Aristotle considers the live-bearing four-footed animals and the egg-laying four-footed to be large kinds even if there is no name in the natural language to designate them.Footnote ⁴⁸ Aristotle elaborates on this front in the Posterior Analytics, where he correlates the following three features: being horned, having a third stomach, and possessing only one row of teeth. All the animals that share these features should be treated as having a common nature even if we lack a name for them.Footnote ⁴⁹

An extended examination of Aristotle’s review of the pre-explanatory strategies adopted in the collection and presentation of the non-uniform parts in blooded animals goes beyond the scope of this chapter. Here I would like to illustrate these strategies with the help of a couple of examples taken from Aristotle’s review of the external parts in live-bearing four-footed animals. The first example is this: the live-bearing four-footed animals have front legs instead of arms, and those that have their front legs divided into many toes use them as if they were their hands (with the notable exception of the elephant, which possesses a special organ, the nose, which serves as a grasping and moving instrument).Footnote ⁵⁰ We find here an instance of how Aristotle proceeds in his review of animal differences. He deals first with the feature (or features) common to the whole kind and then turns to the feature (or features) shared by a particular sub-kind, ending with a discussion of exceptional cases. The discussion of the exceptional cases is not merely a coda motivated by an interest in the strange or the marvelous; rather, it is guided by Aristotle’s concern for offering a review as complete as possible of the zoological phenomena.

Let us now turn to the second example. In his review of the live-bearing four-footed animals, Aristotle tells us that all four-footed animals have legs that are bony, sinewy, and fleshless, and that this is in fact a feature shared by all footed animals. Moreover, all footed animals lack buttocks (most notably, birds). The only exception is the human animal, whose buttocks, thighs, and calves are the fleshiest part of its whole body.Footnote ⁵¹ We find here a good illustration of Aristotle’s interest in generalizing across large kinds of animals. These generalizations contradict Aristotle’s stated intention to study each large kind “separately by grasping its nature individually [χωρίς λαμβάνοντα … ἑκάστου τὴν φύσιν].”Footnote ⁵² Recall that investigating X separately (χωρίς) involves studying X in its own right on the crucial assumption that X is a relatively unified domain of investigation.Footnote ⁵³ If this programmatic statement is taken seriously, we should expect Aristotle to conduct his review of the differences in animals strictly by large kind. And yet Aristotle does not proceed in this way.

To understand why, we need to remain mindful that the large kinds are not his primary, or even most immediate, concern; he is primarily concerned with the commensurate (universal) relationships holding among the relevant animal differences. This concern is dictated by his theory of scientific explanation, which requires the investigator to give explanations at the right level of generality. Aristotle is expected by his own theory to register not only the differentiating features shared by the animals falling under a given large kind but also those that the large kind shares with another large kind (or with a sub-kind of another large kind of animals).Footnote ⁵⁴ This conclusion suggests the following, important observation: the large kinds introduced in HA I 6 are not meant to be a rigid taxonomy adopted for classificatory purposes; they are best understood as a basic division of kinds of animals subordinated to the achievement of the overriding explanatory goals Aristotle pursues in his study of animals.

We come to the same conclusion when we reflect on how Aristotle uses the division into blooded and bloodless animals. Aristotle does not employ this division because he is motivated by taxonomic concerns, but rather because he hopes to pursue his explanatory goals across the entire animal kingdom. To appreciate this point, we need to return to Aristotle’s idea that the absence of blood, blood vessels, and the other related bodily parts entails the presence of functionally analogous parts (they are typically nameless). Two moves are implicit in this idea. The first is this: the parts in bloodless animals are imperfect with respect to their analogous parts in blooded animals; as a result, their description and subsequent explanation is to be conducted by relying on the results achieved in the study of the bodily parts in blooded animals. The second move has to do with analogy, which is the tool that allows us to discover the existence of structural similarities across large kinds of animals.Footnote ⁵⁵ When this tool is first introduced in HA, Aristotle employs the following examples: bone is analogous to spine, nail to hoof, hand to claw, and scale to feather.Footnote ⁵⁶

All these examples are taken from within the group of blooded animals. But the significance of analogy goes emphatically beyond this initial case. Aristotle employs analogy to move not only across large kinds of blooded animals but also from blooded animals to bloodless animals. These two moves jointly control the order of inquiry at the pre-explanatory stage of inquiry: first the differences in blooded animals, then those in bloodless animals. They also control how the differences in bloodless animals are to be approached. These differences are to be introduced and studied based on the results achieved in the study of blooded animals. It does not take long to see that the methodological insight shaping the study of bloodless animals is another incarnation of the methodological principle that the study of the less organized and less determined is to be conducted via the results reached in the study the more organized and more determined rather than vice versa.

The review of the bodily parts in bloodless animals is organized around external and internal parts; but now Aristotle discusses both external and internal parts together for each large kind before moving on to the next large kind. We are not given the reason for this change in the overall strategy. Lennox has suggested that this different organization of the zoological data may be a direct consequence of how the large kinds themselves are fixed.Footnote ⁵⁷ Recall that the large kinds of bloodless animals are four: soft-bodied animals, soft-shelled animals, hard-shelled animals, and animals that have insections in their bodies (i.e., insects).Footnote ⁵⁸ At the outset of HA IV, Aristotle adds a brief description for each of these four kinds. Taken together, those descriptions suggest that Aristotle has come to fix these large kinds with reference to the relative differences that we observe with respect to the softness or hardness of their exterior and interior parts, as well as the shape of their whole body. Hence, it makes sense for him to review these differences large kind by large kind.

I conclude this review of the pre-explanatory strategies adopted in presenting the zoological data regarding the parts of animals by recalling these descriptions. The soft-bodied animals (μαλάκια) resemble blooded animals to the extent that they display soft and flesh-like parts outside and a hard structure inside (e.g., the cuttlefish and the octopus). The soft-shelled animals (μαλακόστρακα) display the opposite bodily arrangement: they have a hard structure outside and soft and flesh-like parts inside (e.g., the lobster and the crab). The hard-shelled animals (ὀστρακόδερμα) have a hard shell protecting a flesh-like inside. The distinction between soft-shelled and hard-shelled animals is traced back to the different nature of their external hard structure, which Aristotle describes in the following way: while the outer structure of the soft-shelled animals is crushable but not crackable, the outer structure of the hard-shelled animals is crackable but not crushable.Footnote ⁵⁹

A couple of questions loom large at this point. The first is how far Aristotle can pursue his explanatory goals by adopting analogy across the entire animal kingdom. The second has to do with the explanatory costs associated with this strategy. Let us focus on the second question first. Analogy helps Aristotle transfer results reached in the study of the more organized and more definite kind of animal to the study of the other kinds of animals. In this respect, analogy orients how Aristotle is going to study what he takes to be comparatively less organized kinds of animals. But it also determines some of the results he is going to establish in the context of his research. Let me illustrate this point with the help of a well-known example. According to Aristotle, soft-bodied animals like the cuttlefish and the octopus are bloodless animals. These animals do not have a heart but something functionally equivalent to it. Aristotle calls this large central part mutis.Footnote ⁶⁰ Today we know better: the cuttlefish and the octopus have three hearts and a bodily organization that is not reducible to the one envisioned by Aristotle. What matters most is not the mistake itself but rather the fact that the mistake is driven by the theory.Footnote ⁶¹ Aristotle does not study these alien forms of life in their own right; he studies them based on his knowledge of blooded animals. An unintended consequence of this strategy is this: Aristotle is unable to appreciate what is unique about animals such as the cuttlefish and the octopus. This last observation raises an important question about the costs of the application of the epistemic principle that requires the investigator to start their inquiry into perishable life from what Aristotle takes to be the most organized and most determinate form of life. This principle has an architectonic relevance that goes beyond the study of animals, but its costs may be assessed by remaining within the domain of animal life.Footnote ⁶²

Let us now turn to the first question raised above. Under the rubric of bloodless animals Aristotle deals with vastly different kinds of animals: insects, soft-bodied animals, soft-shelled animals, and hard-shelled animals. Aristotle cannot be content with highlighting the similarities that hold across these large kinds; he is also expected to engage in a vigorous study of each of those kinds with an eye toward what is specific about each of them. Put differently, the general and the specific are to be integrated into a single account since only the combination of the two gives us perfect, and so scientific, knowledge. There is no reason to think that the study of bloodless animals is meant to be an exception to the rule. But how far, and indeed how well, is Aristotle able to pursue his explanatory goals as he moves away from his study of blooded animals? I will try to answer this question by turning to the διότι-stage of inquiry, with a concentration on Aristotle’s explanation of animal locomotion.

4 Explanatory Strategies in IA

Let us begin by stating the obvious: we do not have a single discussion of all the bodily parts of animals in Aristotle.Footnote ⁶³ In addition to the official treatment offered in PA II–IV, we are given separate treatments of the locomotive and generative parts.Footnote ⁶⁴ While the locomotive parts are studied in the short treatise known with the Latin title of De incessu animalium (IA), the generative parts are discussed in the first book of the Generation of Animals (GA I 3–16). Aristotle does not elaborate on his decision to organize his explanatory work in this way, but we can offer an educated guess on his behalf. Reproduction and locomotion are the two most conspicuous life activities in which animals are engaged. It is surely no coincidence that two of the large kinds that Aristotle isolates in his zoological writings, namely the four-footed egg-laying animals and the four-footed live-bearing animals, are identified by means of their distinctive modes of reproduction and locomotion. Considering this, it is no surprise to discover that the study of the relevant bodily parts involved in animal reproduction and locomotion requires not only extensive but also separate discussions.Footnote ⁶⁵

Here is the opening statement of IA:

Right from the start Aristotle adopts a resolutely teleological approach to the phenomenon of animal locomotion: he promises to deliver a causal explanation of the presence of the locomotive parts by asking for what reason animals have them. Among other things, such an explanation entails finding out the relevant final cause.Footnote ⁶⁷ Moreover, Aristotle promises an explanation not only of why animals have the bodily parts they do but also of why they have them in the way they do. This requires an explanation of why the bodily parts involved in locomotion differ within the same animal (e.g., why front and hind legs in live-bearing four-footed animals bend in opposite ways) as well as across different kinds of animals (e.g., why egg-laying four-footed animals bend their legs in a different way, unique to them – namely, laterally and away from their own body).

What makes the IA an especially interesting case study is the methodological explicitness by which Aristotle first introduces and then uses the relevant theoretical framework to fulfil the promises outlined above. Aristotle provides his reader with three explanatory principles. The first principle is the axiom that “nature does nothing in vain, but it always does what is best from among the possibilities for the substance of each kind of animals.”Footnote ⁶⁸ While Aristotle invokes this teleological principle several times in the course of his natural investigation,Footnote ⁶⁹ he offers its fullest and most precise formulation here.Footnote ⁷⁰ The second principle is the abstract but powerful way of thinking about the living body we have already seen at work in HA. For Aristotle, a perishable living body must be organized in a certain way to support life. This body can display up to six functional dimensions: up/down, front/back, and left/right.Footnote ⁷¹ A perishable living body engaged in locomotion requires all of them. In other words, an upper and a lower part, a front and a back, and finally a right and a left side must be present in a living body equipped with the power to displace itself.Footnote ⁷² The third, and final, principle is about motion: pushing and pulling are the sources of all per se motion from one place to another.Footnote ⁷³ While the first two principles are native to Aristotle’s study of animals, the third originates in Aristotle’s general discussion of motion.Footnote ⁷⁴ This is a relatively unsurprising development. To the extent that Aristotle is about to engage in an investigation of the locomotive parts insofar as they are useful for locomotion, he is concerned not only with certain bodily parts but also with a special kind of motion, namely animal locomotion. So it is perfectly appropriate for Aristotle to remind us (and himself) of a general principle of motion immediately relevant to his project. Unsurprising as it is, this development is not trivial. It shows that Aristotle does not conceive of the study of animal locomotion as a separate, let alone independent, investigation; on the contrary, his investigation is guided by the theoretical framework developed in Phys. I–VIII.

As we proceed in the study of how Aristotle explains animal locomotion, we should bear in mind the following distinction, which also marks the beginning of Aristotle’s actual investigation: bodily displacement takes place either by jumping or by progression (the Greek term is πορεία). While in jumping the living body is displaced all at once, in progression the living body is displaced part by part.Footnote ⁷⁵ This second mode of bodily displacement is Aristotle’s primary focus in IA. The parts of the living body involved in progression are called “instrumental parts.”Footnote ⁷⁶ By adopting this expression, Aristotle indicates that these parts are used by the animal as tools to move its body from one place to another. Finally, Aristotle tells us that an animal that can displace itself must exhibit the following (minimal) level of bodily complexity: one part of its body is acted upon by being pressed, while another acts on it by pressing. An important corollary of this zoological theorem is that without bodily parts nothing can displace itself.Footnote ⁷⁷

Up to this point Aristotle has proceeded at a very general level of investigation, but now he turns to the explanation of the vastly different ways in which animal progression takes place in nature. While some animals progress on land, others do so in water or in air; moreover, some use their feet as an instrument of locomotion, whereas others employ different bodily parts (e.g., wings or fins); finally, there are also animals that can move quickly and efficiently even though they are footless (e.g., snakes). It does not take long to see that a strategy is needed to handle the richness and complexity of the animal world. The strategy devised by Aristotle consists of two steps. The first is the decision to concentrate on the animals that progress on land by means of feet. Aristotle begins his explanation of how animals locomote with an examination of footed animals. He takes the foot to be the paradigmatic instrument of animal locomotion.Footnote ⁷⁸ But this does not solve all our problems given that animals progress on land by means of either two, four, or many feet. In addition, there are animals that progress on land without feet (snakes). Where should we start looking for an explanation of how these different kinds of animals displace their bodies? Aristotle answers this question by developing an explanatory model that works for blooded animals. These animals are either two-footed or four-footed. A direct consequence of this strategy in two steps is that the study of many-footed animals is temporarily bracketed since these animals are all bloodless.

Aristotle’s first and most important goal is to develop a model that explains how blooded animals progress on land by means of feet. This model is subsequently extended (with the relevant adjustments) to explain how bloodless animals progress on land. IA 6 plays a central role in his overall argument. Here Aristotle develops an abstract model for the explanation of how animals that are footed and blooded progress on land. This model is quite general. It is meant to apply to both two-footed and four-footed animals.Footnote ⁷⁹ Aristotle thinks of blooded animals as abstract locomotive units and asks under what conditions they can engage in locomotion. His answer is that these locomotive units must have a common origin of motion, and this origin must be equally well disposed with respect to all the body parts involved in locomotion. All the locomotive units Aristotle envisions in IA 6 are centralized locomotive systems: they have a single source of locomotion that is at an equal distance from all the locomotive parts. Aristotle makes this single source of motion the ultimate cause of bodily displacement.Footnote ⁸⁰

This highly abstract analysis generates the following important result:

To understand the force of this pronouncement, we must keep in mind that Aristotle’s focus is still on animals equipped with two or four feet. When Aristotle says that locomotion belongs only or above all to these animals, he signals to his reader that he is treating these animals as his paradigmatic locomotive systems. Momentarily, we will see how Aristotle goes beyond these paradigmatic locomotive systems. For the time being, it is important to clarify what Aristotle means by “point of motion.” This expression is native to his theory of animal locomotion. With it Aristotle refers to the bodily part that makes contact with the ground (or the surrounding water or air).Footnote ⁸² Given his focus on two-footed and four-footed animals, Aristotle must refer, first and foremost, to the feet of these animals. In his view, these animals progress on land either by means of two or four points of motion. In due course Aristotle clarifies that blooded animals move by means of exactly four points of motion. He does so by replying to the possible objection that birds and humans move by fewer than four points of motion. His reply is that birds use their legs when they fly and their wings when they progress on land, just as humans use their shoulders and their arms when they walk.Footnote ⁸³ But at this stage of his inquiry this conclusion is not yet available.

In IA 6 Aristotle develops a highly abstract account of what a locomotive unit of a certain kind requires to perform its primary function, which is to move from one place to another. Aristotle generates this account for the explanation of how blooded animals progress on land. He subsequently adopts, indeed adapts, it to explain the motion with respect to the place of bloodless animals. Aristotle’s overall strategy becomes clear as soon as we realize that he conceives of bloodless animals as locomotive systems to which additional points of motion are attached to the original four. Moreover, because additional points are attached to the original four, these systems do not display the same level of integration and unity as the one envisioned for blooded animals. One concrete example may help us appreciate how Aristotle approaches the study of bloodless animals. An ant moves by means of six feet. For Aristotle, the ant is neither a two-footed nor a four-footed animal; rather, it is a many-footed animal. Aristotle conceives of the ant as a locomotive system consisting of four + two points of motion. We may think that having more than four feet allows an animal to displace its body in a better, quicker, and more efficient way. However, Aristotle never makes this observation. Instead, his first and foremost concern is to highlight that a unit consisting of more than four points of motion is a less unified, and less well-integrated, locomotive system. It is a less unified, and less well-integrated, locomotive system because Aristotle thinks of this organic unit as having four + n points of motion (where n must be an even number).Footnote ⁸⁴ According to him, there must be a controlling center in this second kind of locomotive system too, but this center cannot be equally well disposed with respect to all the (four + n) points of motion.Footnote ⁸⁵

Aristotle conceives of bloodless animals as less perfect locomotive systems. These locomotive systems are less perfect because they exhibit a lower level of organic unity. This explains why he claims that, unlike blooded animals, these animals can survive for a while if they are severely mutilated. Some of them, Aristotle adds, can even move with the same type of locomotion with which they were moving before they were mutilated. They can do so because they are “just as if they were compounded out of several animals.”Footnote ⁸⁶ Aristotle mentions the centipede. In his view, all animals that have an elongated body and are like the centipede enjoy a weaker type of unity compared to blooded animals. In a few cases, their organic unity is so weak that these animals consist of a few relatively independent locomotive units. In these extreme cases, each locomotive unit can not only survive but also displace itself when it is separated from the others. Or at least this is what Aristotle seems to believe.Footnote ⁸⁷

In a few passages Aristotle engages in the explanation of specific aspects of the locomotion of bloodless animals while he is still concerned with the study of blooded animals, and under a common rubric. He does so because he thinks that this strategy results in an optimal treatment of certain topics. Recall that if a common explanation can be given, then it must be given. Offering a serial explanation instead of a common explanation is no viable alternative in such cases for Aristotle.Footnote ⁸⁸ When we keep in mind this (self-imposed) explanatory constraint, we see why Aristotle is not imposing a too rigid structure on his discussion of locomotion. Aristotle remains flexible to adapt to the complexity of his task, which is to provide as complete a study as possible of how animal progress on land, in water, or in air. A good illustration of his flexibility is the explanation of the role of the tail in flyers.Footnote ⁸⁹ Aristotle finds it convenient to discuss the flight of insects in this very context. He tells us that their flight is slow and is not efficient because they lack a tail. Aristotle must think that dealing with the flight of insects here strengthens his claim that the tail in flyers is functionally equivalent to a rudder.

The occasional, indeed common, treatment of blooded and bloodless animals does not mean that Aristotle has second thoughts about his momentous decision to study how bloodless animals displace their body after his study of locomotion in blooded animals is in place. To see what licenses this approach, we need to recall the methodological insight we have already discussed in connection with HA: bloodless animals do not have blood, blood vessels, and a heart, but they have something functionally analogous to those bodily parts.Footnote ⁹⁰ Analogy is the tool that allows Aristotle to progress beyond his paradigmatic locomotive systems. It is an especially useful tool because it does not reduce or eliminate the complexity of the data, but it gives us a way to map that complexity.Footnote ⁹¹

The explanatory strategy outlined so far allows Aristotle to offer a unified account of animal locomotion that applies to both blooded and bloodless animals without reducing or eliminating the differences between the two groups of animals. This strategy is not without explanatory costs. One is obvious even at a cursory glance: the treatment of locomotion in bloodless animals is disappointingly selective. Bloodless animals become the primary focus of the discussion only at the tail end of IA (i.e., in IA 16–19). Aristotle has already explained why bloodless animals equipped with feet must be many-footed animals. As a result, questions about number of feet and points of motion are no longer addressed. Instead, Aristotle concentrates his discussion on the explanation of how feet are attached to the body of bloodless animals, and on the reason why they bend their limbs in the way they do. Aristotle appears to be especially interested in the case of crabs and karaboi. Both are soft-shelled animals, and this is relevant to the explanation of the way in which they bend their limbs: while crabs use their limbs for progression on the ground, karaboi employ them for swimming.

It is difficult to resist the impression that Aristotle’s answer to the question of how bloodless animals progress would have been significantly different if his answer did not depend on the assumption that all animal progression ought to be understood in light of the results achieved in the study of blooded animals. For one thing, this discussion would have been more detailed. For another, it would have been more convincing. But adopting an alternative route was not a real option for Aristotle. Beginning the study of animal progression by focusing on bloodless animals and moving from there to the case of blooded animals would have been a complete non-starter for Aristotle. It would have amounted to engaging in the study of animal locomotion starting from a less perfect and less developed form of animal life. We are not obligated to follow Aristotle since we do not accept his assumption that bloodless animals represent a less perfect and less developed form of animal life. We can easily object to Aristotle that crabs or octopuses have a very high degree of bodily organization and articulation. And yet this is emphatically not how Aristotle thinks. If one accepts his starting point, one is also compelled to follow his explanatory strategy.

One final aspect of Aristotle’s overall strategy deserves close attention. A definite explanatory pattern can be observed at several junctures of Aristotle’s treatment of locomotive parts. Aristotle always begins his inquiry by explaining the presence of a given locomotive part in certain kinds of animals (e.g., the presence of feet in footed animals), and then explains the absence of the same part in other kinds of animals (e.g., the absence of feet in footless animals). Consider the explanation of how snakes progress either on land or in water. Snakes are blooded animals, so they must displace their body by means of four points of motion. But the only way to move efficiently at four points of motion while having an elongated body is by bending it. So snakes progress by bending their body. They touch the ground (or the water) at two points, one in the front and one in the back of their bodies. Because of their narrow and elongated shape, we cannot see four but only two points of motion. But even if we do not see the four points by means of sense-perception, we can discriminate a functional right from a functional left at each of the two points we see. In other words, snakes are locomotive systems that fit the model elaborated for all blooded animals.Footnote ⁹²

While important, the explanation of how snakes progress by bending their body is only a first, interim result. A full explanation of how snakes move requires an explanation of why they are footless. Snakes are a prima facie exception, and so also a potential challenge, to the basic account of animal locomotion developed for what Aristotle takes to be the paradigmatic case of locomotion – namely, the progression on land by means of feet. By explaining how snakes progress on land (or in water) without employing feet, Aristotle can show that they are no exception to his basic account of animal locomotion. Their motion, properly explained, not only confirms but also strengthens the case for his basic account. In this respect, Aristotle’s discussion of why snakes do not have feet is not a digression from the stated task of explaining why animals have the parts they do. It is rather an integral part of Aristotle’s larger task, which is offering an account of the different modes of locomotion that does full justice to the complexity of the zoological data.

The stretch of text concerned with the explanation of why snakes are footless is one of the best known, and most intensely discussed, because it also contains crucial information on how Aristotle would like to apply the teleological principle that nature does nothing in vain but always the best given the nature of the animal.Footnote ⁹³ Here, suffice it to say that Aristotle takes the properties of having an elongated body and being blooded as the two starting points of his explanation. Aristotle regards them as basic facts that belong to the essence of snakes.Footnote ⁹⁴ In other words, to be a snake is to be blooded and to have an extra-long body. Given this, there are only two theoretical options on the table: either snakes have a foot placed at each of the four points of motion or they have no feet. But having four feet is far from being ideal in the case of an extra-long body. A four-footed animal with an extra-long body would move with great difficulty and very slowly. Snakes are clearly better off without feet. Instead of progressing on land (or in water) by using feet, they move by bending their extra-long body. This turns out to be an extremely effective mode of locomotion.

Among blooded animals, fishes too are footless. In their case, however, the absence of feet is explained by invoking their distinctive mode of life rather than the shape of their body. Like snakes, fishes are blooded animals, so they must move at exactly four points of motions. Unlike snakes, they are marine animals. Since they live in water, they have fins rather than feet. The attachment of their fins, as well as the presence of a tailfin, is explained by exploiting the analogy between swimming and flying, which are regarded as analogous modes of locomotion. At this point, Aristotle has already explained the mechanics of flying. Therefore, he can also adopt the result achieved in the explanation of how flyers move in air to explain how swimmers displace themselves in water.

The above remarks go some way toward mitigating what appears to be a way too brief and cursory discussion of progression in water (swimming). To be sure, swimming is a highly distinctive mode of locomotion, but it is also a mode that can be approached by adopting a comparative method of study. Aristotle’s view must be that the adoption of such a method allows us to know all we need to know about swimming and the bodily tools marine animals employ to displace themselves in water (fins and tailfin). As a result, Aristotle’s strategy in dealing with the different modes of locomotion is this: he first develops an explanation for how animals progress on land by means of feet; he then extends this explanation by considering what is specific about progression in air by means of wings and a tail. By the time he gets to the mode of locomotion characteristic of swimmers, there is not much left to explain. At this point, Aristotle keeps his discussion very short, since he can rely on the explanatory resources already introduced for progression on land (walking) and progression in air (flying). This strategy may also explain why there is no mention of cetaceans. There is nothing distinctive about their mode of locomotion. Like all blooded animals, cetaceans move by means of four points of motion. Like fishes, they displace their body in water by means of fins and a tailfin. By contrast, there is a great deal that is interesting about the way in which cetaceans control their bodily temperature. This explains why they feature in Aristotle’s discussion of respiration but they are absent from his treatment of locomotion.

Aristotle’s study of animal locomotion ends with a discussion of a difficult case: the displacement of hard-shelled animals. Some of these animals appear to be able to displace themselves, but it is far from clear how they move, and whether they are engaged in the form of locomotion that is Aristotle’s primary concern in IA – namely, animal progression. Aristotle approaches this whole topic by announcing that the motion of these animals is a real puzzle (aporia): while they move, it is unclear whether they have a right and a left in their body. His solution to the puzzle consists in considering hard-shelled animals a maimed or mutilated kind of animal. Once more, Aristotle invokes analogy. The implication is that hard-shelled animals have a principle of motion in their body, so we should be able to detect a right and a left side apart in their body. What Aristotle says seems to contradict this. He says that the hard-shelled animals move, but they do so against nature because they are not naturally able to move.Footnote ⁹⁵ Perhaps we can shed further light on this issue with the help of what Aristotle says on the topic of the imperfectly developed animals in the context of his discussion of the locomotive soul. Aristotle claims that imperfectly developed animals move but they do so in an indefinite way.Footnote ⁹⁶ In other words, the capacities of the soul needed for locomotion, as well as the relevant bodily distinctions, are present in them but in an equally indefinite way.Footnote ⁹⁷ Aristotle seems to be saying that the distinction between a right and a left is also present in the hard-shelled animals, but he would add that this distinction is present in them in an indefinite way. Be this as it may, the discussion of the difficult case of the hard-shelled animals is the last topic on Aristotle’s agenda. It is regarded as a sort of coda to the study of animal locomotion for at least two reasons. First, it is no longer a case of progression, which is the stated focus of the treatise. Second, the whole kind is regarded as an intermediate group between stationary and non-stationary animals. Hence, any attempt to explain their motion is naturally placed at the very end of the discussion of animal locomotion.Footnote ⁹⁸

It is time to take stock. Aristotle’s treatment of animal locomotion is far from a random discussion of how animals displace themselves from one place to another. Aristotle adopts an intricate but at the same time principled approach to the topic. His strategy involves recourse to the two epistemic principles we have encountered not only in the Parva naturalia but also in HA. While the first requires us to look for explanations that are commensurately universal, the second mandates that we start our study of locomotion from the form of life that exhibits the highest degree of bodily organization and articulation. Aristotle does not favor one principle over the other but tries to apply both as well as he can. His intricate discussion is also a by-product of his attempt to negotiate the different requirements imposed on him by these principles. The role of apparent exceptions to the rule is a remarkable feature of the overall argument of IA. It does not take long to see that Aristotle’s interest in what is perceived as an exception to the rule, or a borderline case, is entirely dictated by the application of the second epistemic principle outlined earlier. As Aristotle tries to do justice to the complexity of the zoological data, he is required to go beyond the explanation he has developed for his core case: the centralized locomotive unit progressing on land by means of four feet. The discussion of apparently difficult or odd cases contributes directly to this goal. Yet this strategy has its own limitations. The reader of IA comes away with the distinct impression that the locomotion of bloodless animals is, to say the least, understudied. Aristotle does not develop additional principles for the study of the modes of locomotion of bloodless animals; he adapts the principles developed for the study of blooded animals to the study of bloodless animals. The outcome is not entirely convincing. In the end, Aristotle does not appear to be able to do full justice to what is specific, or even unique, about the ways in which these animals move from one place to another.

If we accept the idea that Aristotle’s extant zoological writings are not a complete science but are best understood as a work in progress, we can try to save Aristotle by suggesting that his account of animal locomotion can be revised and supplemented with additional observations. But it is important to stress that the shortcomings highlighted above cannot be fixed unless a drastic change in the overall approach to the study of animal life is accepted. At the very least, Aristotle would have to give up one of the epistemic principles that shape his whole approach to animal life. But there is no textual evidence that Aristotle is ready for such a drastic move. It does not take long to see that such a move would imply rethinking his explanatory strategy from scratch – I mean the strategy to explain the whole domain of animals and not just the phenomenon of animal locomotion.

I would like to end my analysis of how Aristotle explains animal locomotion on a bright note. Notwithstanding all the shortcomings discussed, the explanatory strategy Aristotle adopts allows him to generate a comprehensive map of how animals progress on land, in water, and in air. This result is very important to him. Arguably, it is less important to us today. It is perfectly acceptable for us to focus on a wonderous case of animal locomotion and try to extrapolate from there a few general truths about locomotion. We can easily imagine a scenario in which a great deal of research is devoted to understanding how certain kinds of animals can move easily, almost effortlessly, on a vertical surface. Unlocking the secrets of how these animals perform this feat is a result Aristotle would have surely appreciated. At the same time, he would have considered such a narrow research focus unsatisfying from a theoretical point of view. He would have insisted that we develop a comprehensive account of animal progression rather than try to learn everything about a particular case. More to the point: he would have stressed that such a comprehensive account of animal progression would have to highlight the commonalities existing in nature and would have to give a common explanation for each of them. In the end, Aristotle operates on a more demanding conception of the scientific enterprise. The bar for what counts as epistemic success in this enterprise is set higher for Aristotle than it is for us today.

5 Explanatory Strategies in GA

Aristotle’s GA offers a comprehensive and systematic account of how animals reproduce, including a study of their generative parts. This account employs and connects all the four Aristotelian causes, with a special concentration on the moving (i.e., efficient) cause of the generation of animals. A few additional facts surrounding generation are explained; they include hybridity, birth defects, diseases of pregnant females, and lactation. When we look at what Aristotle accomplishes in GA, we can safely conclude that we are in the presence of one of Aristotle’s most mature, most sophisticated, and indeed most carefully organized scientific writings. In this section, I would like to illustrate some of the explanatory strategies adopted, as well as some of the decisions made, in what turns out to be a long and complex tour de force across five books. My goal is to confirm some of the results reached so far and add to them as is appropriate.Footnote ⁹⁹

Aristotle’s initial move in GA consists in a three-part observation: in many cases, animals come to be from (1) the coupling of (2) a male and (3) a female.Footnote ¹⁰⁰ This is our initial observation – what we observe when we first look. It is one observation (rather than three) because the items that constitute it are interdependent. Neither male nor female nor coupling, as we observe them in most cases, makes sense without the other two. This initial observation establishes our first explanandum. Since we want to explain a process that begins from the coupling of the male and the female, our explanation must include an explanation of the male and the female qua generative principles. This comprises an explanation of their respective generative parts. In other words, an optimal study of animal reproduction must comprise a study of the reproductive capacities of the male and the female, which in turn requires a reference to their generative parts. The outcome is a single, complex argument that consists of a study of generative parts followed by a study of how animals come to be, which first requires us to deal with the moving cause (alias principle) of animal generation:

There are at least two ways to understand this programmatic passage. On a first reading, the two studies outlined correspond to the two main sections of GA I – namely, a study of the generative parts (GA I 3–16) followed by a study of the moving principle of generation (GA I 17–22).Footnote ¹⁰² As an alternative, the parts that contribute to the generation of animals are not only the non-uniform parts but also semen, menses, and milk. If we adopt this second reading, what Aristotle has in mind when he refers to an account of the generative parts is a fuller account than the one advanced in GA I. This fuller account is not over until at least the discussion of milk offered in GA IV 8.Footnote ¹⁰³ I adopt this second reading because it has the exegetical advantage of showing that GA II is a natural development of a discussion that starts much earlier in GA I.Footnote ¹⁰⁴

To understand how GA V contributes to an account of animal generation, we must place an emphasis on the following (often overlooked) fact: nothing Aristotle says in the first four books establishes that the generative process stops at birth. In fact, this generative process comes to an end only with the coming to be of a mature organism. We can restate this important insight by recalling a well-known Aristotelian dictum: “it takes a human being to generate a human being.”Footnote ¹⁰⁵ By Aristotle’s lights, generation is a natural and continuous process from two fully developed human beings (the parents) to another fully developed human being. Consequently, the study of generation need not (in fact, cannot) stop at the birth of the offspring but must continue with a logical focus on the mature organism.Footnote ¹⁰⁶

If this reading is correct, a first consequence of the initial move – namely, the tripartite observation that many animals come to be from the coupling of a male and a female – is the decision to begin the study of animal generation with an account of the generative parts of the female and the male. Another important consequence of this move is the decision to postpone the explanation of animals that (Aristotle thinks) reproduce spontaneously. This explanation is deferred until the main lines of the account of sexual reproduction are firmly in place. This means that the study of spontaneous generation is given only at the very end of GA III (i.e., GA III 11). This study is treated as a coda to the main discussion. Along with a few insects, this study attends to the generation of hard-shelled animals and other stationary animals that Aristotle considers close to plants. Finally, it is also important to stress that this study is conducted within the theoretical framework developed for the explanation of sexual generation. Aristotle is quite forthcoming about this aspect of his explanatory strategy at the outset of GA I 2:

This is exactly how Aristotle’s argument unfolds in GA: it begins with a study of sexual generation and ends with a discussion of the generation of those animals the coming into existence of which is spontaneous. It does not take long to see that this procedure entails a reversal of the explanatory strategy adopted by Aristotle’s predecessors. Most of them worked within a cosmogonic framework. It was quite natural for them to take spontaneous generation as their model, and indeed their starting point, for an explanation of how life emerged from the earth. Among other things, this style of explanation had the advantage of relating their explanation to the ancient myths of the autochthonous birth of cultural heroes.Footnote ¹⁰⁸ By contrast, Aristotle does not believe that life emerged from nonlife. His view is that life is a necessary feature of the natural world order.Footnote ¹⁰⁹ More to the point: animal life is one of the two basic ways in which life necessarily manifests itself here on earth.Footnote ¹¹⁰ The only way to explain how animal life perpetuates itself is to start from sexual generation, which becomes the paradigmatic type of animal generation. As a result of this strategy, spontaneous generation is dislocated from the center of the argument of GA. It ends up being equivalent to an exception to the rule that must be explained based on the results reached in the study of the central case, namely sexual generation. Beginning an explanation of animal reproduction from spontaneous generation would be equivalent to trying to explain animal life starting from bloodless animals. This would be a complete non-starter for Aristotle.

In GA III 11 spontaneous generation is explained as a two-stage process that is analogous to sexual reproduction. In sexual generation we have a two-stage process: first the material principle is formed in the female; then it is acted upon by the male principle. In spontaneous generation we have an analogous complex situation. The heat in the surrounding environment, by a process of concoction, produces something analogous to the female residue. In rotting earth or earthy water, we find what Aristotle calls pneuma, and in pneuma we find vital heat. Whenever some of the latter gets enclosed, it leads to the generation of the animals. The language Aristotle uses to describe this process suggests that the agent is envisioned as an analogue to the immediate moving cause in the process of sexual reproduction. Unlike the power transmitted by the male, the vital heat is a non-species-specific potential for life. This means that eventual differences in the outcome cannot depend on differences in the nature of the potential for life. Rather, they depend on differences in the nature of the enclosed material. In the case of the hard-shelled animals produced in the sea, the earthy water of the sea becomes the shell as the result of a process of solidification analogous to the one that occurs in the case of bodily materials such as bones and horns.Footnote ¹¹¹

A third and final consequence of the initial move is the need to understand the respective contribution to the generative process of the male and the female, including the extent to which each contribute seed (σπέρμα).Footnote ¹¹² It is quite telling that, already in the opening chapter of GA, the study of seed is predicted to be of the utmost importance in understanding the distinctive roles of the male and the female in the coming to be of the offspring:

In view of this anticipation, it is not surprising to find out that Aristotle turns to the study of seed as soon as he has completed his study of the reproductive parts. The question whether both the male and the female emit seed is linked to the question whether the seed comes away from all the body. Apparently, this thesis was quite widespread in antiquity. It was linked to the view that both the male and female contribute seed (σπέρμα).Footnote ¹¹⁴ For Aristotle, disarming the dual-seed theory is equivalent to disarming pangenesis and vice versa. This is his first step toward a positive account of the nature of seed, and toward establishing how the male and the female contribute to the generative process.

Aristotle is centrally concerned with these questions in the second part of GA I (GA I, 17–23). His starting point is to search for the nature of the seed. First, he establishes that the seed is a residue, and then that it is a useful residue at its final stage – blood concocted and somehow particularized. Thereafter Aristotle turns to the female contribution. He establishes that the blood-like secretion produced in the female (the menses or καταμήνια) is a spermatic residue, having its nature because of the colder (and thus weaker in the ability to concoct) character of the female. By Aristotle’s lights, the menses are the matter from which the offspring will come into being. It is only at this point that Aristotle turns to the male contribution with the stated goal of establishing what it is. He argues that the male does not contribute any matter but rather an active power or a potential (δύναμις) that is based on certain movements (κινήσεις) conveyed by the semen.Footnote ¹¹⁵ Aristotle uses the craft analogy to argue that the male contributes the form by means of its role as the source of motion. In those animals that emit semen (σπέρμα), nature uses the latter as a tool just as a craftsman uses specific tools to produce a certain outcome.Footnote ¹¹⁶ At this point, the main lines of Aristotle’s reproductive hylomorphism are in place.Footnote ¹¹⁷ More directly, Aristotle adopts hylomorphism in the second part of GA I (GA I 17–23) to explain the different contribution of the male and the female to generation. While GA I is not meant to stand on its own, the book provides us with an explanatory framework to proceed in the actual investigation of animal reproduction as it can be observed in the natural world around us.

6 Conclusion

Nothing short of a full-length monograph would do justice to what Aristotle has accomplished in his systematic study of animals. My goal in this chapter was a more modest one. I wanted to give the reader an idea of how this study is guided by certain explanatory and methodological concerns. I reached two main results. I argued that the ὅτι-stage of inquiry as collected in HA is shaped by the way in which these explanations are to be offered at the διότι-stage. In this sense, the ὅτι- and the διότι-stage of inquiry are interdependent and complement each other. The second result is that at the pre-explanatory and also at the explanatory stage of inquiry Aristotle follows a definite strategy encapsulated in the catchphrase “first what is first.” When we look in detail at how Aristotle proceeds in his practice of scientific explanation, we see that he first identifies a paradigmatic case, which is the most determined and most articulated kind of animal, and then he tries to explain the non-central cases by applying the results achieved in the study of this paradigmatic case. This strategy is implemented by recourse to analogy. I have argued that it comes with some explanatory costs, since significant aspects of the non-paradigmatic cases are not convincingly accounted for.

Aristotle concentrated on the study of animals to the exclusion of plants. But this does not mean that he was not interested in the study of plants. His scattered remarks on this topic suggest that he was quite knowledgeable about plants.Footnote ¹²⁹ Aristotle thought that a study of plants was best pursued as a separate explanatory project. Unfortunately, we no longer have what Aristotle wrote on plants, so we are not in a position to evaluate how he transitioned from the study of animals to the study of plants or how he implemented a systematic study of plants. But we do have an impressive corpus of writings on the topic of plants left by Theophrastus. It is time to turn to these writings. My main goal in the next two chapters is to see whether, and eventually to what extent, the research on plants conducted by Theophrastus is shaped by the explanatory and methodological concerns that animate Aristotle’s study of animals.