According to a philosophical commonplace, Aristotle defined human beings as rational animals. When one takes a closer look at the surviving texts, however, it is surprisingly hard to find such a definition. Of course, Aristotle repeatedly stresses that he regards rationality as the crucial differentiating characteristic of human beings, but he nowhere defines the essence of what it is to be human in these terms. What is more, Aristotle’s abundant remarks about human nature are scattered throughout his texts, and he offers no systematic treatise on human beings.

The notion that human beings are imperfect in many ways and that their perfection requires special means is commonly associated with Plato rather than with Aristotle. Plato not only treats the objects of the material world as deficient copies of their immaterial and eternal models or ‘forms’ in general, but displays a pessimistic view of human nature in particular. It must, of course, remain forever a matter of debate in how far Plato shares the view about the poor natural endowment of human beings that he lets the famous sophist Protagoras pronounce in the dialogue he named after him (Prot. 320c–324d). But there are good reasons for assuming that Plato at least shared the sophist’s diagnosis concerning the natural human endowment in two respects: (i) in comparison with other animals, humans are born in a particularly vulnerable state. They are, by nature, provided neither with food nor with cover nor with the means of self-defence: ‘naked, unshod, unbedded, and unarmed’ (Prot. 321c). According to Protagoras’ myth, the poverty of human nature is compensated for only by the ‘gift of Prometheus’: it is the invention of fire and of certain crafts that ensures the provision of life’s necessities. (ii) The Promethean gift of the crafts alone would have been insufficient to ensure the survival of the human race. The formation of communities, as a means of defence and mutual support, required Zeus’s special gift of ‘political art’. Without the notions of justice and shame, humans would inevitably have destroyed each other. Just like the technical arts, according to the myth in the Protagoras, the social arts are not part of the natural human endowment. Their development presupposes training and teaching over many years. It is a task that requires the joint efforts of the entire community.

According to a philosophical commonplace, Aristotle defined human beings as rational animals. When one takes a closer look at the surviving texts, however, it is surprisingly hard to find such a definition. Of course, Aristotle repeatedly stresses that he regards rationality as the crucial differentiating characteristic of human beings, but he nowhere defines the essence of what it is to be human in these terms. What is more, Aristotle’s abundant remarks about human nature are scattered throughout his texts, and he offers no systematic treatise on human beings.

The claim that human beings are by nature political animals is one of the most fundamental of Aristotle’s Politics, and, understandably, it has received a lot of attention.1 One very interesting, and fruitful, trend has appealed to the biological works to illuminate this famous thesis.2 This strategy has brought to light a broad conception of politicality which is exhibited by humans and other gregarious animals, including bees, wasps, ants, and cranes.3 As scholars have rightly emphasised, political animals in the broad sense collectively pursue a common end via a differentiation of roles or tasks. Different broadly political species have different common ends. Bees promote the good of the hive, wasps that of the nest, and so on. Nonetheless, every broadly political species has a shared end – really, a shared way of life – that all of its members promote in different ways. This is true of humans, although their way of life specifically involves forming and sustaining poleis, which makes them political in another, narrower, sense as well.4

If someone has considered the study of the other animals to lack value, he ought to think the same thing about himself as well; for it is impossible to look at the parts from which mankind has been constituted—blood, flesh, bones, blood vessels, and other such parts—without considerable disgust.

Aristotle establishes in this passage a strong link between a substance’s ergon and its aretē.2 Ergon is commonly translated as ‘function’.3 In many cases, it is even conceived of as something’s function or purpose in the literal sense. At least this is how Socrates appears to define the term at the end of the Republic, book I, claiming that a substance’s ergon refers to the work for which the substance is the sole or best instrument (352e and 353a). Socrates’ reference to the pruning knife to cut vines is the paradigm case. While their shoots could also be cut with a carving knife, or a chisel, or many other tools, Socrates explains how no tool does it better than the pruning knife, which is made for the purpose. Thus, according to him, we are entitled to define the ergon or function of the pruning knife to be the act of pruning. As Socrates further states (353b), everything that has an ergon also has a distinct virtue (aretē) or vice (kakia). While it is through its virtue that it performs its ergon well, the opposed vice makes it perform its ergon poorly (353c). Socrates claims this model holds not only for artefacts such as the pruning knife, but more generally: In the same way it is attributed to the knife, it is also attributed to the eye, the horse, and the human being. Since artefacts are considered paradigmatic, we may call the Socratic model the functionalist conception of virtue. Virtue here means a substance’s fitness to serve a distinct purpose that is assigned to it; it means aptitude, serviceability, or even instrumentality.4

An important question regarding Aristotle’s moral anthropology is whether or not human beings can become perfectly good. Should we consider Aristotle’s paradigmatically virtuous agent (who appears under the different names of spoudaios, phronimos, agathos, epainetos, epieikēs, or kalos kagathos) as an ideal figure, i.e. as someone who permanently and infallibly executes morally correct actions? Is his character irreversibly transformed into a state of perfection? Might we describe his inner condition as that of perfect psychic harmony? Or is it true, as some interpreters claimed, notably Shane Drefcinski and Howard Curzer,1 that the virtuous man (Aristotle never mentions female candidates) can act badly? Does he perform, at least to some extent or from time to time, slightly suboptimal actions, or perhaps even more: evil deeds? If Aristotle’s moral anthropology allowed for some imperfection or even considered imperfection as an unavoidable part of the human condition, it would be close to the view of the philosophos held by Plato. If, on the other hand, Aristotle defended an ideal of absolute perfection, he would be somewhere contiguous to the Stoic concept of the sage (sophos).

In the following chapter I would like to draw the reader’s attention to a passage in Aristotle’s De Caelo that, to my mind, reveals some of Aristotle’s deeply rooted presuppositions about the role and the standing of human beings within the cosmos. Strictly speaking, the passage, which is in book II, chapter 12 of De Caelo, is not meant to discuss anthropological questions; it rather unfolds a comparison between celestial bodies and living beings. In the course of this comparison it turns out, somewhat surprisingly, that out of all the kinds of celestial bodies, it is the planets whose nature is most akin to that of mankind. Since explanation, both in Aristotle and in real life, proceeds from what is better known and more fundamental to what is less known and in need of an explanation – in the given case from living beings, such as plants, animals, and humans, to celestial bodies – the very idea of this comparison presupposes that there are certain facts about human beings that are relatively uncontroversial and are not themselves in need of explanation. Owing to this general setting we can be confident that the characteristics of human beings that Aristotle adduces are thought to be general truths about the human condition – although he does not really engage with the details and the implications of these supposed truths.

Human beings are not the only social animals, according to Aristotle. He doesn’t even think they are the only political animals. And in the beginning of book 8 of the Nicomachean Ethics, he suggests that non-human animals can also be friends with each other: most of them entertain friendships with their young, as well as with other members of their own species.1 So, is friendship not uniquely human either?