I

From its early days, Hippocratic medicine conceived itself as an eminent technē (‘craft’, ‘science’, ‘art’, ‘productive knowledge’, ‘expertise’) based on practice and observation. Although the empirical orientation of Hippocratic medicine remained mostly undisputed throughout the Classical period – despite numerous points of dissension among Hippocratic writers on other matters – this self-conscious epistemological ideal was difficult to reconcile with actual clinical practice. Before human dissection was first permitted at the medical institute of Alexandria (circa 270–260 BCE),Footnote ¹ Greek doctors were not allowed to dissect human corpses due to religious and cultural taboos, while the cutting of the patient’s skin was typically restricted to superficial incisions and drainages in exceptional circumstances.Footnote ² As a result, the inner structure of the human body and its mechanisms were largely terra incognita for Greek physicians. The boundary, both symbolic and physical, that separated the internal constitution of the body from the external world easily accessible to the senses was established by the skin.

This paper investigates how exactly Greek doctors could cope with this limitation while simultaneously vindicating their aspirations to empirical knowledge. In examining this tension and its implications for Greek medicine, my purpose is twofold: first, to discuss some neglected aspects of Hippocratic epistemology concerning the role and scope of sense perception in ancient Greek medicine; second, and relatedly, to address and neutralise an apparent inconsistency among different Hippocratic texts in connection with this question. To this end, the discussion centres upon, although it is not restricted to, three medical treatises: On the Art [of Medicine] (= de Arte), On Ancient Medicine (= VM) and Nature of Man (= Nat. Hom.).Footnote ³ I have selected these medical writings because of their particularly instructive views on medical epistemology but also by reason of their alleged inconsistency on some cardinal epistemological questions. More precisely, while some of them state that what is accessible to sense perception represents but a small portion of the entire medical domain (eg. de Arte 11; see [T3] and [T4] below), others place great emphasis on the empirical character of medical knowledge on the grounds that medicine concerns itself only with what is manifest to sense perception (eg. VM 1 and Nat. Hom. 1; see [T1] and [T2] below).

Two possible ways to deal with this inconsistency, each inspired by familiar strategies in Hippocratic scholarship, come immediately to mind: we can leave the inconsistency as it stands (eg. by attributing the authorship of these treatises to different medical writers), or we can resolve the inconsistency itself by showing that, upon closer examination, it is only apparent. My contention is that, questions of authorship notwithstanding, there is compelling textual evidence available for endorsing the latter view. Most importantly, a proper justification of this position will require us to investigate the epistemological foundations of Hippocratic medicine. All these medical authors, I argue, resort to the same methodological approach – a skilful combination of analogical characterisation and inferential reasoning – in order to make room in medicine for what escapes sense perception – by remaining hidden under the skin – without having to abandon their claims to empirical knowledge.

II

It has been suggested that Greek medicine was already well established as a reputable technē towards the early fourth century BCE and perhaps even earlier.Footnote ⁴ While it is now generally agreed that the bulk of the Hippocratic Corpus was most likely written before the end of that period, the fact remains that, by this time, Greek medicine was still struggling to secure a respectable place among other forms of knowledge in antiquity. As parties to a stimulating debate on the epistemic status of medicine, the initial challenge that Hippocratic authors had to face was not a minor one: to prove, despite the scepticism of its critics, that medicine did deserve the title of technē. Ironically, the most valuable textual evidence of the rather fragile situation of Hippocratic medicine during the Classical period was produced by Hippocratic writers themselves. Although medical authors never developed an epistemology proper, let alone a philosophy of science, the Hippocratic Collection is peppered with incisive remarks on the distinctive character of medical knowledge and its relation to other sciences. The interest of Hippocratic authors in epistemological questions emerged in part as a response to different groups of detractors who denied that medicine was a genuine technē. Their criticism was based on three kinds of considerations: (C1) medicine’s dependency on natural philosophy (eg. VM 1-2; Nat. Hom. 1), (C2) its alleged lack of success and accuracy in clinical diagnosis and therapy (eg. de Arte 4; VM 9) and (C3) its close connection with mythology and ‘religious healers’ (eg. Morb. Sacr. 1). Particularly relevant for present purposes are (C1) and (C2).Footnote ⁵ Let us first address (C1).

In order to become a respectable science, a preliminary obstacle that medicine was required to overcome was its supposed incompatibility with a basic constraint implicit in the Greek conception of technē. This was the idea that a technē is a cohesive system of knowledge which applies to a specific domain of objects, thus distinguishing itself from other domains.Footnote ⁶ In spite of its apparent simplicity, this formal constraint became especially problematic for Hippocratic writers, as critics disputed that medicine was indeed an autonomous body of knowledge with a distinctive subject matter. Remarkably, the debate was not only between Hippocratic doctors and their detractors, but also among Hippocratic authors themselves. While some of them suggested that the foundations of medical knowledge must be derived from natural philosophy, others considered medicine to be a self-standing form of knowledge, viz., a technē in its own right. To put it roughly, the first group of physicians contended that patterns of health and disease were determined by, and at the same time exhibited close parallels with, cosmic elements and processes that doctors were required to know in order to treat patients. This is the Hippocratic tendency that permeates the medical doctrines of Fleshes (Carn.) and Breaths (Flat.), and most famously On Regimen (Vict.).Footnote ⁷

On the opposite side, another prominent group of Hippocratic authors strongly resisted this philosophical trend in medicine. Questions about human physiology may have played some role in the justification of their views, the central thought being that the human body is made of humours rather than the physical elements that constitute the cosmos (Nat. Hom. 1-2,VI.32-36 L. = 164-170 J. and 4,VI. 38,19-40,2 L. = 172,13-174,15 J.). This was not the most critical point at stake, though. Their main line of criticism was premised on epistemological considerations relative to the scope and nature of medical knowledge. The Hippocratic treatise On Ancient Medicine provides important evidence on the substance of the issue. From the opening of the text, its author sets out to vindicate the epistemological autonomy of medicine by refuting those medical writers who endorse a reductive approach to medical aetiology to the effect that all causes of human disease and death can be explained in terms of a few hypotheses [hupotheseis]. The full passage reads thus:

As can be gathered, the author’s main objection is levelled at some medical writers of the time that make use of a certain hypothesis in order to account for the ‘primary cause’ of disease and death. It is considerably less clear, however, (i) what exactly we are meant to understand by ‘hypothesis’ in the passage, and (ii) on which grounds the author condemns the adoption of hypotheses in medicine.Footnote ⁸ Let us tackle each question separately.

As for (i), the scholarly consensus has it that ‘hupothesis’ in [T1] stands for ‘postulate’ or ‘assumption’ of some sort, yet what exactly this means in the overall economy of the treatise remains obscure.Footnote ⁹ Illustrations of it are introduced in [T1] by means of a binary opposition between the corresponding qualities of the four elements commonly discussed in pre-Socratic cosmology: ‘hot or cold, or wet or dry’. This provides a first hint, however vague, of the breadth and scope of the hypothesis to be rejected: it is not a mathematical but physical assumption.Footnote ¹⁰ We are explicitly told that such assumptions are employed by the author’s adversaries as basic explanatory principles meant to account, apparently without much success, for the primary causes of disease and death. From this standpoint, the explanandum of the hypothesis under consideration cannot be further argumentative moves within a broader dialectical interchange, but more specifically, as [T1] makes abundantly clear, biological phenomena, or specific descriptions thereof, such as human diseases and ultimately death.Footnote ¹¹

In practical terms, since what requires explanation are diseases and other physical ailments – this is our explanandum in [T1] – it is safe to conclude that the hypotheses under attack must correspond to assumptions intended to explain their causes. The exact scope of such hypotheses is, however, considerably less clear. On a narrow interpretation, we are dealing with specific cases each of which may demand a different hypothesis: H_1: ‘disease x is caused by what is hot (as opposed to what is cold)’, H_2: ‘disease y is caused by what is cold (as opposed to what is hot)’ and so forth. Nonetheless, the fact that the author regularly refers to the hypothetical method of his opponents in terms of a singular hypothesis [hupothesis], as opposed to plural hypotheses [hupotheseis],Footnote ¹² strongly suggests that he is operating with some sort of higher-order explanatory principle with the following structure: ‘every disease is caused by (either an excess or deficiency of) what is hot, cold, wet or dry’. Once granted, this principle is subsequently employed by his adversaries as a foundational assumptionFootnote ¹³ to account for the causes of all diseases (several illustrations of how they proceed are mentioned in VM 13 and 15).

In connection with (ii) (ie. on which grounds the author condemns the adoption of hypotheses in medicine), the author of VM raises not one but several objections to the aetiological model of his rivals, yet all of them are the expression of one and the same general concern: to dismiss the idea that medical knowledge must be derived from the sort of speculative thinking that characterises natural philosophy and cosmology. All in all, we can identify two main lines of criticism: one is intended to question the very existence of the items which are hypothesised by his opponents, and the other one challenges their alleged usefulness for clinical practice. As regards the first objection, it is true that the mere existence of phenomenal qualities, such as hot, cold, dry or moist in the natural world, is never explicitly denied by the author, but he takes his opponents to endorse a somewhat stronger thesis, namely, that there is ‘absolute’ or ‘pure’ hotness, coldness, dryness or moistness, which is therefore not mixed with anything else. The author complains, however, that no such thing has ever been discovered (VM 15.1,I.604, 14-16 = 137,15-17 J.). His objection is open to two possible readings, and the two of them are consistent with, and indeed supportive of, the authors’ characterisation of the hypothesis under attack as ‘empty’.Footnote ¹⁴ We can take it to mean that there is no such thing as, say, a food that is absolutely hot, cold, dry or moist, because all food, while having one or more of these qualities to some degree, must also be astringent, sweet and bitter, which are the real qualities [dunameis] in food the author considers to be aetiologically relevant for human health (VM 15). Alternatively, we can also interpret the assertion that there is no such thing as what is purely hot, cold and so forth, as entailing that the objects that we perceive in everyday experience, especially food, do not contain any of these qualities in their pure form but rather mixtures of them where some elements predominate over the others (cf. Nat. Him. 2, VI.36 L. = 168,9–170,1 J.).

The second line of criticism, as just noted, centres on the putative use that his opponents’ hypothesis may have for medical practice. That the main issue at stake is eminently practical becomes clear in later sections of the treatise where the author confesses to be at loss (‘Aporeō d' egōge…’, VM 15.1, I604,13 L. = 137,12 J.) to understand how supporters of the hypothetical method in medicine can actually treat [‘therapeuousi’] patients on the basis of it. For suppose that the hypotheses in question are right, so that (say) hotness and coldness are indeed the real causes of health and disease. If one hot food is astringent, while the other one insipid, it would therefore make no difference whether doctors administer either of these to the patient, for the aetiologically relevant factor is the same in both cases. However, the effect of what is hot and insipid on the body is manifestly different from the effect of what is hot and astringent (VM 15.2–4, I.604-606 L. = 137-139 J.). Consequently, hotness and coldness cannot be causally active factors in determining patterns of health and disease, and hence references to them alone in this context are pretty much ‘useless’ for clinical purposes (15.2, I.606, 1 L. = 138,1 J.). In contrast, according to the author’s own aetiological and physiological model, it is not such qualities that are responsible for disturbing or restoring a patient’s health but rather a complex combination of the ‘quality’ [‘dunamis’] or ‘strength’ [‘to ischuron’] of other physical properties in food: its degree of sweetness, bitterness, acidity and saltiness, inter alia. Because such qualities are also present in the humours inside the body, they interact directly with the bodily constitution of human beings, thereby increasing or decreasing the volume of each humour (VM 14.4, I.602,12-15 L. = 136,8-16 J.).

Truth be told, this reasoning seems to expose the author to the same objection that he raises to supporters of hypotheses in medicine.Footnote ¹⁵ Yet he is emphatic to point out that, unlike the groundless hypotheses of his adversaries, the different qualities present in food, which do have the power to alter the concentration and distribution of humours inside the body, were discovered long ago by physicians thanks to direct empirical observation (VM 14.4, I.602,8 L. = 136,8 J.) and reasoning (14.3, I.600,19 L. = 135, 15 J.). Particularly suggestive in this respect is the author’s later remark that the only real measure medicine has at its disposal is the ‘perception [aisthēsis] of the human body’.Footnote ¹⁶

Regarding the identity of the author’s adversaries, despite multiple conjectures entertained by scholars in the past, the truth is that neither the list of potential hypotheses is meant to be exhaustive, nor are criticisms directed at one specific medical writer. Rather elusively, the author acknowledges to be taking issues with anyone who sets out to account for the primary causes of diseases in terms of the specific qualities just discussed ‘or anything else they want [to postulate as a hypothesis]’ (= [T1]). As the argument unfolds, it becomes apparent that his adversaries are those doctors who wrongly ‘tend towards philosophy’ in medicine (VM 20.1, I.620,10-11 L. = 146,4 J.) – not incidentally, Empedocles is singled out as one of his main targets (VM 20.1, I. 620,10 L. = 146,4 J.). The aetiological model under attack is reminiscent of other Greek medical traditions but also of other Hippocratic writings with a more philosophical orientation. I have in mind Alcmaeon of Croton’s seminal account of health as the balance of bodily constituents such as wet, hot, dry, cold and other ‘indefinite number’ (Aëtius 5.30.1 = DK 24B4) or perhaps Philistion’s later expansion of Alcmaeon’s medical ideas in the light of Empedocles’ cosmology (Anonymus Londinensis XX.25-30). But even within the vast range of treatises that give shape to the Hippocratic Collection, we find many medical writers who championed theories and ideas that were closer to the medico-philosophical doctrines of these medical thinkers than to those of other Hippocratic authors.Footnote ¹⁷ This influence is most clearly traceable in several medical texts where patterns of health and disease are ultimately couched in terms of the excess or deficiency of the four elements, or at least some of them, inside the human body (eg. Vict. I. 32 and 35; Carn. 2; Flat. 3). To mention one representative illustration, the author of On Breaths claims to have demonstrated that his hypothesis [hupothesis] (ie. that pneuma, which permeates the whole universe, is the cause of every disease in men) holds true (Flat. 15, VI.114,13-19 L. = 124,11-125,1 J.). This is precisely the sort of hypothesis that is explicitly rejected in [T1].

It has long been acknowledged by scholars that, as far as their general views on the nexus between medicine and philosophy are concerned, there are some remarkable affinities between On Ancient Medicine and Nature of Man. They become apparent already in the opening section of Nature of Man where the author begins his argument with certain reservations about those who investigate the nature of man by going ‘beyond its relationship to medicine’. While promising to deal with foundational questions of human physiology, they postulate the existence of invisible elements in the human body, such as fire, water, air or earth, as its primary constituents. The passage is worth quoting at length:

Unlike [T1], the author of Nature of Man is taking issue with two different factions of medical writers. In agreement with [T1], the first group is represented by those medical thinkers who contend that the human body is made of elements such as air, fire, water, or earth ‘or anything else whose existence in human beings is not manifest’. Members of the second group, on the other hand, are said to advocate some form of ‘physiological monism’ (the label is mine) to the effect that the human body is ultimately made of one single element and nothing else (Nat. Hom. 2, VI.34,11 L. = 166,15J.). While the author’s criticisms to the first group is mostly driven by epistemological considerations, physiological monism is rejected on aetiological grounds. On the basis of his own aetiological account, but also with the further assistance of some basic ontological assumptions about the nature of change in general, the author offers a rebuttal of physiological monism by means of a simple and elegant reductio: if physiological monism is true, and the human body is indeed composed of one element, and one element only, then bodily ailments, which are ultimately physical alterations, would not be possible. This is due to the fact that any form of change requires a plurality of co-existing things, or at least more than one, to take place (Nat. Hom. 2, VI.34,17-36 L. = 168,4-11 J.). But, alas, bodily ailments do exist, so the objection runs, so the human body cannot be composed of one element only.

More significant for the task at hand are the implications that the author’s response to the first group of opponents carries for his overall epistemological outlook. As indicated by [T2], his rejection of element theory in medicine is premised on an empirical ideal of medical knowledge that is also vindicated by [T1]. More precisely, we are told that medicine does not, or should not, concern itself with what is inaccessible to sensory experience, as suggested by doctors under the influence of pre-Socratic cosmology. In the language of VM, the scientific foundations of medicine cannot be laid out by an empty hypothesis about things which are unobservable and unverifiable. Formulated in positive terms, medicine’s only epistemic standard is identified with ‘the perception of the body’ (cf. note 16). Similarly, the author of [T2] disagrees with the physiological theory of his adversaries not only because they go ‘beyond its relationship to medicine’, but also, and relatedly, because the physical elements which that theory claims to identify in the bodily constitution of man are ‘not manifest’ or simply ‘invisible’ (cf. note 18).

It is true that [T2], unlike [T1], makes no explicit allusion to any kind of suspicious hypothesis as its main target. But this certainly does not prevent these two medical writers from sharing substantive views about the nature and scope of medical knowledge. Both authors challenge the assumption that medicine must be grounded on general principles borrowed from natural philosophy. They do so, moreover, by emphasising what exactly must be rejected, namely, the application of four-element theory to the understanding of human physiology (= [T2]) and aetiology (= [T1]). As a result, they are united by a common understanding about how one should conceptualise the fragile connection, if any, between medicine and natural philosophy. In contrast to other medical writers, these two Hippocratic authors regard medicine to be an autonomous form of knowledge which is therefore equipped with these two a distinctive subject matter and method. Finally, the two of them also emphasise that the incorporation of philosophical assumptions into the medical domain is not only unnecessary but also detrimental. According to [T1], those who champion aetiological theories based on empty hypotheses, such as the hot, the cold, and so forth, ‘make a mistake’, whereas [T2] states that those who think of the human body as composed of fire, water, earth or air are incapable of attaining ‘correct knowledge’. In sum, both authors are committed to the idea that medicine is already a self-standing science and that the intrusion of natural philosophy into medicine is disadvantageous for its consolidation as a genuine technē.

III

Another Hippocratic treatise with similar apologetic intentions is On the Art. From its opening sections, the author shows himself to be interested in vindicating the epistemological status of medicine as a genuine technē (de Arte 1-3, VI.2-6 L. = 224-227 J.). At least in this regard, his main goal aligns well with the general aspiration of both VM and Nat. Hom. Unlike VM and Nat. Hom., however, he adopts a rather different strategy that centres on the rejection of (C2) rather than (C1). That is to say, instead of addressing the charge that medicine is not a genuine science by reason of its epistemic dependency on natural philosophy, the author deals with the rather different objection that medicine is not a proper technē but just a matter of sheer ‘chance’ [‘tuchē’]. In response, the author repeatedly emphasises that the goal of medicine is to secure a highly prised human good, health, thereby preventing all sorts of ailments that would otherwise affect human beings (de Arte, 4.2-3, VI.6,11-18 L. = 227,12-19 J.; cf. VM 3.4-6, I.576,1-20L. = 121,15 – 123,8 J.). Because doctors are said to accomplish that goal by following strictly rational procedures, their expertise can hardly be the fruit of sheer chance (de Arte. 3.2-3 VI.4,16 – 6,5 L. = 226,12-227,5 J.; 4.2-4, VI.7,16-27 L. = 227,12-19; 5, VI.6-10 L. = 228-230 J.).

Remarkably, despite his insistence on the scientific and purposive character of medical knowledge, the author of de Arte is also ready to admit that its subject matter, the human body, constitutes a major impediment for the scientific consolidation of medicine, while at the same time assuming, somewhat paradoxically, that it also accounts for its very existence. On the one hand, as noted earlier, the study of the human body endowed medicine with epistemic unity, so to speak, thus allowing it to demarcate itself from other departments of knowledge, in particular natural philosophy.Footnote ¹⁹ On the other hand, however, the human body was simultaneously viewed by Hippocratic doctors themselves as an obstacle for the accreditation of medicine as a genuine technē. In the author’s own words, while it is possible for other crafts to work with material that is ‘visible’ as well as ‘malleable’ (de Arte 11.7 VI.22, 2-14 L. = 238,20-239,14 J.), the human body is neither one nor the other (12.1, VI.22,15 – 24,3 L. = 240,1-3 J.).

To make sense of the otherwise puzzling assertion that the material medicine works with is not visible, some brief historical remarks are in order. As noted in the section I, the most important parts of the body for medical inquiry, the organs and cavities under the visible skin, could not be directly perceived by doctors due to certain sociocultural constraints, in particular the Greek prohibition to practise dissection on human corpses. As a result of some religious and moral taboos, which constituted a great impediment for the progress of human anatomy in antiquity, the human corpse came to be seen by the Greeks as a source of pollution, but also, paradoxically, as a symbol of purity.

The human corpse ie. was a source of pollution without being itself polluted.Footnote ²⁰ In practice, this popular belief translated into a prohibition to cut open human corpses, the skin thus becoming not only a symbolic barrier for doctors but also a physical frontier which they were not allowed to cross. For Hippocratic authors, the cutting of the skin was limited to incisions or quick drainages of bodily fluids.Footnote ²¹ Of special interest for present purposes are the concrete implications that a sociocultural constraint of this kind carried for medical practice and methodology.

Such is roughly the historical background against which the apology of medicine that we find in de Arte must be understood. After conceding to his adversaries that the subject matter of medicine is indeed exceptionally difficult to deal with, the author of de Arte introduces some qualifications so as to disallow the further conclusion that, on such grounds, medicine is not a genuine technē. To substantiate his position, he outlines an original nosological taxonomy.Footnote ²² The two central passages where this short but instructive taxonomy is presented are worth quoting:

In line with [T3], diseases can be sorted into two chief categories: some of them are ‘not hard to see [or detect]’, and others are ‘not easy to see’. Whereas the former are further characterised as ‘evident’ or ‘visible’ (‘en eudēlōi’; see also ‘ta phanera’ at 10.1, VI.16, 16 L. = 235, 11 J.), the latter are described as ‘not visible’ (‘adēla’, 11.1, VI.18,14-15 = 237,5 J.).Footnote ²³ The physical limit that separates one group of diseases from the other is the skin: evident diseases ‘erupt’ on the surface of the skin, whereas nonevident diseases remain hidden inside the ‘bodily cavity’.Footnote ²⁴ We thus obtain a division between ‘external’ (= manifest) and ‘internal’ (= hidden) diseases. At the same time, this nosological distinction is paired with a further epistemological contrast: external diseases are accessible to sensory experience, but internal diseases are not (= T3 with de Arte 11.1, VI.18,14-15 L. = 237,4-7 J.). In cases where the disease escapes direct sense perception, doctors are forced to rely on the subjective and unreliable opinion [doxa] of the patient. To the extent that this doxastic component is unavoidable in the diagnosis of internal diseases, medical infallibility is not to be expected in such circumstances (de Arte 11, VI.20,13-15 L. = 238,5-7 J.). Yet infallibility is not an unreasonable epistemic standard in the diagnosis of external diseases, for direct sense perception of them is indeed possible (de Arte 9.4, VI.16,10-14 = 235,3-8 J.).

In virtue of the explicit apologetic tenor of de Arte, there is something disconcerting about these Hippocratic texts. Upon further reflection, they seem to undermine, rather than uphold, medicine’s status as a technē. As shown by both [T1] and [T2], a recurrent strategy adopted by Hippocratic doctors to counteract the objections of sceptics was to emphasise the firm empirical basis of medical knowledge: unlike other putative sciences that purport to study the invisible without much success, medical knowledge rests ultimately on what is evident to sensory experience. But this is precisely the statement that [T3] appears to challenge, while also adding, moreover, that medicine actually deals with the invisible for the most part (!). The clear terminological affinities shared by these medical writings make this doctrinal discrepancy all the more puzzling: after distinguishing diseases which are ‘not hard to see’ from those which are ‘not easy to see’ in [T4], the author of de Arte portrays the former as being ‘manifest’ or ‘visible’ [ta phanera]. The author of Nat. Hom, on the other hand, argues in [T2] that medicine is not concerned at all with ‘what is not manifest or visible’ [mē phaneron]. The main difficulty is, of course, that [T4] employs this terminology to remind us that the visible constitutes only a small portion of the entire medical domain, while Nat. Hom. adopts it to arrive at exactly the opposite conclusion: the invisible lies completely outside doctors’ sovereignty.

Against this background, therefore, we are left with two seemingly inconsistent positions about the nature and object of medical knowledge. This result makes one wonder how, and whether, we can reconcile the nosological outline of de Arte [= T3, T4] with the medical empiricism of both On Ancient Medicine and On Nature of Man [= T1, T2], but also with the views of several other Hippocratic writings: eg. Surgery (Off.) 1, (III.272 L.); Epid. 6.8.17 (V.530 L.); VM 9 (128.9 J. = I.588-90 L.); Vict. I.23 (VI.494-96 L.). Shall we just give up and leave things as they stand, namely, as yet another inconsistency in the Hippocratic Collection that may be explained on either chronological or geographical grounds? I do not think so. But most importantly: there is no need to think so either.

IV

In order to neutralise this apparent inconsistency at the heart of the Hippocratic Corpus, I suggest drawing attention to one of the most fascinating – and yet least explored – topics of Hippocratic epistemology: the ‘mental crossing’ of the skin.Footnote ²⁵ On the plausible assumption that the human body is not exempted from the regularities we perceive in the natural world, doctors aimed to infer [tekmairesthai] how organic processes inside the body take place by looking at the physical world outside it.Footnote ²⁶ By combining empirical observation, analogical characterisation and inferential reasoning,Footnote ²⁷ doctors thought it possible to apprehend the unobservable on the basis of the observable. Digestion eg. was occasionally assimilated to cooking (eg. VM 11.1, I.594,6-11 L. = 131,11-18 J.; cf. Aristotle Mete. IV. 381b6-9); in direct auscultation, the internal sounds of the thorax, especially inside the lungs, were sometimes compared to seething vinegar [Diseases (Morb.) II.61, VII.94,16-17 L.], sometimes to a rubbing leather (Morb. II.59, VII.92,4-6 L.) and in gynaecological treatises, the smell of vaginal discharges was likened to that of fetid rotten eggs [Female Diseases (Mul.) II.115, VIII.248,3-4 L.]. Save for the last reference, where bodily fluids are immediately accessible to sensory experience, in all these illustrations, medical writers describe the inner, hidden body in terms of what is perceived outside it. Further yet, a more instructive form of analogical characterisation was facilitated by the dissection of animals at early stages of comparative anatomy [eg. The Sacred Disease (Morb. Sacr.) 11.3-4 Grens. VI.382 L. with note 1]. In any case, regardless of whether the relevant comparanda are human organs and artefacts (or natural objects), or else human and animal organs, the methodological procedure at play remains one and the same. This procedure is aptly summarised by the author of VM: ‘one must learn these [things inside the body] from evident things outside the body’ (VM 22.3, VI.626,14-15 L. = 149,15-16 J.).

Physicians’ journey into the bodily hollow or cavity (cf. note 24) was everything but smooth, though. Initially, where possible, they are encouraged to patiently wait until the disease becomes manifest to sense perception through signs or symptoms (‘sēmeia’, de Arte 12.2, VI.24,6 = 240,9 J.). The pathological sign crosses the skin from the inside out, thereby leaving a path (‘hodos’, de Arte 12.5, VI.24,13 = 240,17 J.) behind itself which can then be followed backward by doctors. In this way, doctors can venture, if only with their minds, into the darkness of the human cavity, moving in the opposite direction to that of signs: from what is perceptible to what is not. Truth be told, not all modes of perception enjoyed the same epistemic status. Anticipating later theories of sense perception advocated by Greek philosophers (eg. Plato, Timaeus 47a; Aristotle, Metaphysics 980a19-27), it was not uncommon for Hippocratic writers to regard sight in particular as the most informative sense of all – to the extent that the patient must first be seen, it was also the first sense to be used in medical diagnosis [Prognosis (Prog.) 2 II.112,12-14 L.]. The fact that all the main Hippocratic passages examined thus far, [T1]–[T4], demarcate the medical domain in terms of what is visible, as opposed to what is invisible or obscure, bears witness to the priority that was given to visual experience by Hippocratic writers. But this certainly did not prevent medical authors from emphasising the pivotal role of every sense in medical diagnosis: touching, smelling, hearing and even tasting were all considered to be able to capture pathological signs.Footnote ²⁸ Sensory experience was only a starting point, however. As noted earlier, in medical diagnosis, analogical characterisation was also subsequently involved, and the imperceptible to any of the senses was often cashed out in terms of what is accessible to at least one of them.

Now, for all its indispensability, an isolated sign rarely, if ever, carries a clear and definite meaning on its own. Hence medical diagnosis is inherently synthetic: when diagnosing, doctors bring together perceptible signs to make a synoptic judgment based on them (Prog. 17 II.158,1-2 L.; 25 II.188,9-10 L.). In order to decipher the true message of individual signs,Footnote ²⁹ doctors are encouraged to cultivate a certain sensibility to grasp the true meaning of each sign against a broader background of concomitant factors: the natural constitution of the patient, the presence or absence of other signs, the temporal manifestation of each sign, geographical location and even seasonal changes were all critical data that doctors were required to ponder in order to grasp the message that a specific sign is conveying in a given situation. Understanding the symptom in its full singularity, as opposed to just perceiving it, demanded from doctors far more than merely collecting the raw material provided by sensory experience. This explains why the author of de Arte does not regard the task of doctors as that of passive recorders of nature’s manifestations but rather as expert ‘interpreters’ or ‘translators’ [‘hermēneiōn’] of them (de Arte 12.6, VI.26, 4-5 L. = 241, 7-11 J.) – a task that cannot be performed without careful reasoning (‘logisamenoi’, de Arte 7.3, VI.10,24 L. = 231,12 J.; cf. Prog. II.150,13: ‘xullogizomenon’; see also ‘tōi logismōi’, 11.3, VI.20, 7 L. = 237,17 J.) and intelligence (‘gnōmē’, de Arte 11.2, VI.20,3 L. = 237,12 J.).

It proves suggestive that the operation of human intelligence, whereby internal diseases are apprehended, is explicitly assimilated by the author of de Arte to visual perception. In his own words: ‘what eludes the sight of the eyes is captured by the sight of intelligence [gnōmēs opsei]’ (de Arte 11.2, VI.20, 2-3 L. = 237,12 J.). Remarkably, in describing the doctor’s cognition of the invisible in the language of visual experience, the author obliquely ratifies his commitment to an empirical model of medical knowledge by extrapolating the certainty and reliability of sense perception, in particular sight, to the diagnosis of internal diseases. An extrapolation of this kind also reveals his confidence in the cognitive possibilities of the human mind: knowledge of the inner body, however, inferential and mediated may be, remains nonetheless as a reliable form of cognition, comparable indeed to direct visual perception of physical objects. Crucially, unlike ordinary instances of sense perception, the doctor’s perception of the disease is portrayed as a cognitive achievement rather than as a pre-reflective encounter with the natural world. Where the layman sees just a multiplicity of signs, the doctor identifies sensory manifestations of one and the same cause: the pathology itself (Vict. I.11, VI.486,12 = 134,21 Joly-Byl; see also de Arte 12.3, VI.24,7-9 = 240,10-14 J.). Among all Hippocratic treatises, it is in de Arte where we encounter the clearest formulation of the epistemological parallel between sense perception and mental representation that will prove to be so decisive for later Greek epistemology.Footnote ³⁰

V

In view of the foregoing considerations, it is worth asking how systematic and consistent Hippocratic epistemology was. In particular, does On the Art [= T3–T4] really contradict the empirical ideal of medicine championed by both On Ancient Medicine and On Nature of Man [= T1–T2]? We have seen that the terminology employed by these medical treatises is strikingly similar, if not identical. Yet it is precisely in virtue of this terminological affinity that certain inconsistencies among these medical texts become all the more striking: while both [T1] and [T2] oppose the view that medicine deals with what is not evident or manifest to the senses, both [T3] and [T4] state that what is manifest/visible represents only a small fraction of the medical domain on the grounds that most diseases are not manifest to sense perception. Is it possible, in sum, to bring these Hippocratic texts together under a consistent epistemological line of thought? I think it is. At least in this specific respect, I suggest, these medical writings do not subscribe to mutually exclusive views.

Indeed, when the author of Nat. Hom. declares that the invisible lies outside medical jurisdiction, he is evidently not committing himself to some sort of naïve empiricism conforming to which medicine, and medical physiology in particular, is exclusively concerned with what is visible. Not only would such a verdict be manifestly at odds with actual medical practice, but it would also contradict his own views about medical methodology and the significance ascribed by them to the analogy between the visible and the invisible (Nat. Hom. 6, VI.44-46 L. = 178-179 J.).Footnote ³¹ Most importantly, even if we were to leave matters of consistency aside for a moment, nothing of what is stated in [T2] can authorise the further conclusion that there is no more to medical knowledge than what is accessible to sensory experience. Upon closer inspection, the author’s claim that doctors do not deal with the invisible [= T2] is not made without qualification but rather under a significant proviso, namely, that such things are invisible in human beings (‘en tōi anthrōpōi, Nat. Hom. 1.5, VI.32,5 L. = 164,6-7 J.). While the qualifier ‘in human beings’ may seem redundant at first, it actually discloses critical information on the semantic extension of the set of things that are said to be invisible in [T2]: they are invisible… in human beings! This stipulation enables us to account for the otherwise puzzling reference to the four elements of early pre-Socratic philosophers as particular instances of things that cannot be seen (Nat. Hom. 1 VI.30,3-6 L. = 164,6 J.). Were this stipulation absent, a natural objection would be that the four elements of early cosmologists are clearly visible in the physical world. Nonetheless, this objection is neutralised from the beginning precisely because the author’s assertion is not that such elements are unqualifiedly invisible, but rather that they are invisible as physical constituents of the human body. Evidently, from the fact that the four elements cannot be seen in the human body – for there is nothing to be seen in the first place – we are not allowed to infer that there is no room whatsoever in medicine for the invisible. More precisely, invisible or internal diseases are certainly not ruled out by [T2]. In a similar vein, when the author of VM rejects the adoption of philosophical hypotheses in medical aetiology, the items that are hypothesised by his adversaries correspond to the phenomenal qualities of the four elements (ie. hot, cold, dry and moist; VM I.570,3-5 L. = 118,3 J.). Once again, the fact that all such qualities may be perceptible in the natural world is clearly beside the point. And yet, such hypotheses are said to be utterly useless and based on empty speculation – indeed, the sort of hypotheses that are characteristic of ‘invisible matters’ [‘aphanea’].Footnote ³²

To conclude, in the light of the foregoing considerations, we can see there is no real incompatibility among these medical texts but actually a fairly coherent line of reasoning on the method and scope of medical knowledge. For the medical doctrines that the authors of Nat. Hom. and VM oppose do not match those that the author of de Arte endorses: while de Arte is describing, in a favourable light, a transit from the visible to the invisible thanks to the ‘sight of intelligence’, Nat. Hom. and VM reject the method of physicians who start out with dubious assumptions about what cannot be perceived in the human body. For the author of VM, such assumptions correspond to foundational ‘hypotheses’ or ‘postulates’ about the putative existence of pure or unmixed forms of hot or cold or wet or dry in the body which escape sense perception, thus having no place in clinical practice. Similarly, central to the main argument of Nat. Hom. is the idea that doctors should not concern themselves with what is not perceptible in the human body as a starting point of medical inquiry. Those who proceed in this fashion are committed to the antithesis of the analogical method vindicated by both VM and Nat. Hom. whereby the invisible is eventually grasped by first perceiving what is visible. The invisible, in sum, does have a respectable place in medicine: it is a point of arrival rather than departure.