Fermentation versus concoction: two problematic terms

In a 1640 letter to Henricus Regius, Descartes exposes his view on the ‘triple concoction’ taking place in the human body, with its ‘perfect’ final outcome – at the end of these three concoctions – being the blood, which arises through a ‘pulsific ebullition’: the chyme ‘in verum & perfectum sanguinem per ebullitionem pulsificam commutatur’.Footnote ¹²

‘Concoction’, a word used since antiquity for a variety of processes, some of them identifiable as common fermentations, was an Aristotelian term.Footnote ¹³ For Aristotle, ‘concoction (πέψις, pepsis) is maturity, produced … by a thing's own natural heat’.Footnote ¹⁴ Afterwards, thanks to the success of Galenic medicine, this term became associated with the traditional description of digestion, which is exactly the sense employed by Descartes. Yet Descartes's thinking about digestion and its inherent concoctions seems to have undergone changes over time, with the account of the threefold concoction being abandoned in the latter half of the 1640s.Footnote ¹⁵ The secondary literature recognizes the problem but does not currently give us a decisive answer to why. Peter Anstey, who addresses the issue more precisely, concludes that Descartes must have ‘considered a more sophisticated account of ebullition as non-essential’.Footnote ¹⁶

I propose a different hypothesis – that this observed change in Descartes's approach has to do with an intentional shift away from the term ‘concoction’ and towards an emphasis on the more feasible, for the purposes of mechanism, ‘fermentation’. Specifically, I argue that Descartes turned more to ‘fermentation’ precisely because he believed it would help him to avoid the teleology present in ‘concoction’, and that this marks a specific development in his thought about digestion and physiology in general.

To underpin this hypothesis, let us examine the use of ‘fermentation’ in this same letter. First, in the account quoted above, fermentation is the process by which the chyle is digested and transformed into chyme, a process said to take place in the liver: ‘In hepate … chylus … fermentatur, digeritur, & in chymum abit.’ A few lines later, explaining the same phenomenon, Descartes attracts our attention with a perhaps surprising word choice: ‘I say the chyme (juice of the stomach) in its turn is fermented in the liver and digested, that is, exactly as this word is usurped from the Chymists, it is altered by means of a delay.’Footnote ¹⁷ ‘Usurped from the Chymists’ (‘prout hoc verbum a Chymicis usurpatur’): this is hardly a neutral expression. Instead, it signals Descartes's discomfort with using a new term. Most likely, the ‘usurped’ term is ‘to ferment’, or more precisely in this case, ‘to be fermented’ (lat. fermentari): ‘fermentari puto’ – ‘I claim that it is fermented’, or ‘I claim it to be fermented’. I take this passage to mean, by paraphrase: the chyme is ‘fermented’ in the sense in which Descartes's ‘Chymists’ use this word, meaning entailing an alteration that is due to a delay. This delay was a relatively common feature of sixteenth- and seventeenth-century accounts of fermentation, and it most likely mirrored the everyday experience that any fermentation does not happen instantaneously, instead necessitating a certain time to spread within the mass of the alterandum. Alteration, too, belonged to the conceptual arsenal of the chymists, referring to various changes in matter, including the more special case of transmutations (sanguification being, for medical alchemists, a specific case of transmutation). However, chymistry could hardly claim a monopoly on this term; ‘alteration’ was a common term related to mixtures and transformations of matter used from antiquity and common in medieval Scholastic writings.

Whether Descartes refers to fermentari or alterari propter aliquam moram as the ‘usurped’ term, my hypothesis is that here Descartes may have had a sense that he got too close to chemical philosophy by making use of expressions commonly employed in the latter. In this letter to Regius, he still uses ‘concoction’, a Galenic and Aristotelian term, while cautiously adding ‘fermentation’ to his arsenal of concepts. It is possible that later in the decade he decides to drop ‘concoction’ altogether in order to distance himself from those more traditionally minded of his contemporaries who still employed ‘concoction’ (a majority). The main issue with ‘concoction’ is that it entailed a strong teleological component ever since Aristotle, where it plays a role in nutrition, but also – more decisively – in growth, which rarely fails to act teleologically. ‘Fermentation’, on the other hand, might have seemed – even as late as 1640 – a term that was gaining ground, but as yet unformed enough to be appropriated by Descartes's mechanical philosophy more easily than it would have been to take ‘concoction’ and strip it of its long-standing teleology.

Paradoxically, the chymists themselves in the meantime would appropriate the term ‘fermentation’, gradually implementing its use for processes formerly designated ‘concoction’. The extensive use of ‘fermentation’ in this sense – replacing ‘concoction’ – was originally Paracelsian, hence not particularly new in the 1630s. Paracelsus, in his turn, had likely drawn inspiration from medieval sources, such as Pseudo-Avicenna's and Roger Bacon's writing on ‘ferments’ of a spiritual nature that encoded an ‘active power’.Footnote ¹⁸ But the ones who promoted this particular usage most proactively were indeed the later chymists, such as Joan Baptista van Helmont and Franciscus Sylvius. The chymists’ inclination to use ‘fermentation’ was initially linked to their rejection of traditional Galenic and Aristotelian ideas, which argued that heat and the ensuing concoction were the efficient cause of digestion. Putting forth ‘fermentation’ was a way of rejecting the joint model of concoction and heat, because this new, spiritualized concept of fermentation did not necessarily involve heat. This was the case for Van Helmont, and it is due to the alchemical tradition of spiritual ferments as formal causes: if transmutation (or any irreversible transformation of matter – hence not mere mixtures) can happen solely due to ferments which act as formal causes, then the role of heat is being made redundant. For Van Helmont, who renounced using ‘concoction’ in a physiological context, digestion was composed of a series of fermentations, each phase with its own specific ‘ferment’.Footnote ¹⁹ It was those acid ferments, and not heat, which promoted digestion. Descartes, however, is unlikely to have been inspired by Helmontian medicine directly as concerns fermentations, for by contrast to Van Helmont and other earlier chymists, Descartes did assign an important role to heat.

Following Descartes, other later users of the fermentation model would reintegrate heat into their accounts as well; as committed corpuscularians acting within a more established paradigm, their distance from the spiritual ferment tradition enabled them to readopt heat, which can function as an efficient cause of material nature in the absence of any causation of spiritual nature.

Like the chymists, Descartes, too, aimed to distance himself from the ancient medical tradition, which is why a ‘fermentation’ purged of traditional Galenic overtones appeared more attractive. By and large, in this sense Descartes's endeavour of conceptualizing physiology in terms of mechanical philosophy competed with that of the chymists, whose ‘fermentation’ carried different overtones, especially a non-material dimension. The two paradigms developed more or less in parallel over several decades of the seventeenth century. A mere handful of authors had employed ‘fermentation’ in their explanation models for physiology before Descartes; as shown by Clericuzio, these were authors indebted to the chymical philosophy.Footnote ²⁰ Yet in the end, Descartes is as keen to distance himself from the chymical authors as he wishes to break with Galenic medicine.

To sum up, I argue that by using ‘fermentation’ Descartes attempted to circumvent the traditional teleological component of ‘concoction’, present ever since Aristotle. He ventured to use a term which, however, came with its own complications: an association with chymistry and a centuries-old teleological baggage.Footnote ²¹

Descartes, heat and alchemy

The issue of Descartes and alchemy continues to spark controversy. In the following, I will argue that while Descartes's use of fermentation bears some similarity to contemporary chymical language, either it is a superficial borrowing of a term, or the underlying shared premises can be traced back to common ancient and medieval sources. In both situations it is difficult to make the case for direct influence or inspiration.

To support this stance, I bring three arguments. First, I trace Descartes's reluctance to use the term ‘fermentation’, ever since the 1630s. Second, I show that Descartes's idea of how exactly fermentation happens differs substantially from that of chymists, in two main respects: the physicality of the process and the role of heat (contrary to the early chymists, Descartes does assign heat a leading role). Third, I show that the heat which chymists do make room for in physiology, and which has been confounded with Descartes's ‘fire without light’, is a concept of different lineage: its origin is celestial, while Descartes's heat originates in fermentation understood as innate agitation of particles.

First, in Traité de l'homme, Descartes explains how digestion works, portraying it as a physical and fermentative process:

Digestion, hence, happens by a process that is analogous to chemical ones, but the explanation for this process is of an exclusively mechanical nature. In the analogy with hay, Descartes refers to what we recognize as a fermentation while avoiding the term itself.

Another instance of unnamed fermentative processes as background for physiology is found in the Discours sur la méthode, where Descartes exposes his theory regarding how God created human beings, first as bodies similar to our own, then placing into the heart a fermentational fire. Here, he repeats the hay analogy and adds another one from winemaking:

In this case Descartes places fermentation in an even more crucial position where fermentation processes are made to account for the principle of life. Thus fermentation becomes a cornerstone element in Cartesian medicine and philosophy of nature. Notwithstanding this point, once again he does not name fermentation explicitly.

This omission is an issue which has barely received any attention.Footnote ²⁴ Most translations, in fact, reintroduce the term ‘ferment’ at the end of this passage, in spite of its absence (though fermentation as we understand it today was certainly part and parcel of the envisioned process).Footnote ²⁵ In my view, however, the glaring absence of the term is most likely intentional on Descartes's part. My hypothesis is that Descartes purposefully circumvents the term, in an attempt to lay the basis for an understanding of fermentation that differed from the chymists’ contemporary understanding of it – and hence the philosopher's wish to distance himself accordingly. Consequently, my proposed translation of the above passage differs from the existing ones. On the wines, Descartes simply writes ‘on les laisse cuver’; I propose to translate cuver as ‘to mature’, a term which would have fallen under the semantic realm of the Aristotelian ‘concoction’. (This, in turn, raises the issue of natural teleology, which I will address later in more detail.)

Descartes's reluctance towards the term may have a prehistory in his correspondence with Plempius. In an exchange of 1638, Descartes explicitly proposed ferments and fermentation as an explanation model (by analogy) for the beating of the heart.Footnote ²⁶ Plempius, however, unceremoniously held the ferment to be a ‘figment’, and presumed that Descartes was hard-pressed because there was no other appropriate concept.Footnote ²⁷ Descartes was unpleasantly surprised.Footnote ²⁸ But later omissions of the term may be read as an indication of a reappraisal on his side.

Second, Descartes employs a concept of fermentation different from that held by contemporary chymists, both because it is explained physically (geometrically) – such as digestion described above – and because it does entail heat. We have seen earlier the role of heat being central to the debate on fermentation versus concoction. Traditionally, (Galenic, Aristotelian) concoction could not happen without heat. Alchemists like Van Helmont saw in fermentation a way to reject concoction by saying that fermentation did not necessitate heat, since it was caused by a ferment of spiritual nature.Footnote ²⁹

While Descartes also aimed to set his theory apart from the ancients, he did ascribe a central role to heat in digestion as well as in cardiac movement, fuelled by a ‘fire in the heart’. This corresponds to the idea of innate heat (the food being ‘heated up of itself’), a concept intimately connected since antiquity with living warm-blooded bodies.Footnote ³⁰ Hippocrates and Aristotle, as well as Galen, considered it fundamental, with Aristotle in particular holding heat to be a result of the action of the vegetative soul.Footnote ³¹ Descartes, intent as he is on doing away with the vegetative soul, claims that it is the result of fermentations, which hence form ‘the first principle and motor of our whole [bodily] machine’.Footnote ³² The fluids heated by fermentation glide in between the parts of the food and break it down, a process reminiscent of Plato's ‘cutting’ quality of heat in Timaeus.Footnote ³³ The breaking down of food by means of their fermentative agitation causes the particles to ‘descend gradually toward the passage through which the coarsest of them must exit’.Footnote ³⁴ Thus, as is known, the dynamic distribution of particles of food during the fermentative process of digestion is determined by the geometry of these particles. The coarser ones go downwards and exit, while the more ‘subtle’ ones move upwards through the liver.Footnote ³⁵ It is true that a similar process of separation of coarser and finer elements is also fundamental for spagyria, the alchemical art.Footnote ³⁶ In alchemy, the separation of the more coarse, impure parts from the purer and lighter ones was the task of the Paracelsian ‘internal alchemist’, or in Van Helmont's version, of the ‘Archeus’.Footnote ³⁷ This separation is a feature of the ‘philosopher by fire’, philosophus per ignem, the alchemist.Footnote ³⁸ Descartes, however, is unlikely to have at this point been inspired by either Paracelsus or Van Helmont directly; rather, this operation of purposeful separation, as one of the steps that make up the digestive process, was an idea that permeated thinking about digestion both in ancient and in modern views. More likely, it originates in empirical observation of how common fermentations happen, both in nature and in industry, forming the blueprint of a process which alchemy rather inherited than invented. It was so ubiquitous in antiquity that it was used to describe processes as diverse as from the artisanal production of metals and wines to various physiological processes such as blood formation.Footnote ³⁹ These processes, moreover, were put into correlation by analogy. In a famous example, Galen compares digestion and blood formation to the fermentation of wine:

For Galen, the liver hence transforms the chyle into blood by means of a coction comparable to the fermentation of wine.Footnote ⁴¹ While Descartes reduces this to what Aucante calls a ‘processus purement physique’, he is describing roughly the same process (or ‘workflow’, to use a contemporary term), but explaining its causes differently.Footnote ⁴²

Third, the conceptual origin of Descartes's ‘fire without light’ has been subject of controversies, being recently deemed alchemical.Footnote ⁴³ Was, then, Descartes's fire (heat) inspired by that of the alchemists? I would argue that this is not the case, since the nature of Cartesian fire was elemental, not celestial like the innate heat of the alchemists. Moreover, the idea to place this fire in the heart, while also employed by alchemists, was part of a common repository of ancient ideas.

For Fernel, as well as for the seventeenth-century alchemists who followed him, innate heat – different from elemental fire – had its source in celestial bodies such as the sun.Footnote ⁴⁴ This does not mean that the heat of the body comes to it from the outside (which would be understood as ‘external heat’), but rather that the body's innate heat is related to solar heat by its nature. But while Fernel claimed that he was deriving his theory from Aristotle, the Stagirite had been ambiguous about the origin of vital heat.Footnote ⁴⁵ As Gad Freudenthal has shown, Aristotle's animal heat was not of celestial origin; rather, the ‘astrologization’ of Aristotle's heat came to be with Averroes, and it is likely this line of thought that Fernel and alchemists adopted.Footnote ⁴⁶ Fernel had hence offered an interpretation of Aristotle through the prism of Arab alchemy.Footnote ⁴⁷

That Descartes respected Fernel's authority in general is hardly surprising, given the latter's status as one of the most authoritative physicians of the previous century.Footnote ⁴⁸ Respect, however, need not coincide with agreement. Descartes, as we have seen, held the heat in the body to be a result of fermentations, which he understood as mechanical processes. This view echoes a different Aristotelian suggestion, namely that innate heat might have its source in food, an idea more fully developed by Galen.Footnote ⁴⁹

In the issue of the ebullition of the blood in the heart, Descartes departs from received notions – both from Fernel and from the scholastics – of the pulsation of the heart being governed by a vital faculty that manages the heat; instead, Descartes posits that the pulsific movement comes from fermentation alone.Footnote ⁵⁰ As he writes in the letter to Plempius in reference to cardiac movement, ‘there is a sudden expansion and as it were boiling up of the blood in the heart. And what, I ask you, could cause a greater or more sudden change in a body than just such a boiling up and fermentation?’Footnote ⁵¹ For Descartes, therefore, physiological heat (the ‘fire without light’) is elemental, not celestial, and in this respect it differs from alchemical ideas of innate heat.

As for the main location of the innate heat being in the heart, while the idea was also entertained by Fernel, it looks upon a long tradition stretching back into antiquity.Footnote ⁵² It is in accordance with Plato's Timaeus and with the author of the Hippocratic treatise on the heart (Peri kardies).Footnote ⁵³ Aristotle even compares the heart, as a source of heat, to, ‘as it were, a hearth, where that which kindles the whole organism shall reside’.Footnote ⁵⁴ Later, Galen fully embraced the idea, and in On Anatomical Procedures he called the heart ‘the seat of passion and source of “innate heat”’.Footnote ⁵⁵ This is not to be taken as an indication that Descartes's inspiration would necessarily come directly from these particular ancient sources, but rather that this idea is part of a much older repository that he shares with all of his contemporaries, including the alchemists.

The perils of analogy: importing natural teleology into physiology

Descartes notoriously rejects teleological explanations of non-living as well as living bodies, insofar as they are devoid of mind; yet his relationship with teleology is not straightforward.Footnote ⁵⁶ As his apprehension regarding ‘concoction’ shows, he hoped to avoid its teleology. Instead, he employed ‘fermentation’ at a crucial joint of his argument: he placed ‘fermentation’, conceived mechanistically, in a position to replace ‘concoction’ as well as the traditional ‘souls’ of Aristotelianism and Galenism. But was ‘fermentation’, in this life-sustaining role, a viable candidate for Descartes to be able to eliminate teleology from physiology? More specifically, can Descartes reconcile the natural teleology inherent in ‘fermentation’ with his concept of a non-teleological living body? I argue that this is only partially the case.

Some background is necessary. In recent years, scholars such as Stephen Gaukroger, Karen Detlefsen and others have significantly advanced our understanding of Cartesian teleology as it applies to living systems. Detlefsen recognizes that ‘Descartes routinely makes use of teleological-sounding explanations in his biological works’ and that this creates tension in Descartes's philosophical system.Footnote ⁵⁷ She claims that for Descartes ‘living bodies – human or other – have no teleological natures at all, and so the teleological-sounding explanations that Descartes uses in his biological works pose an insurmountable difficulty for him’.Footnote ⁵⁸ The form that teleology takes in Descartes is, Detlefsen proposes, Aristotelian (broadly, ‘natural’ teleology): ‘Aristotelian teleology is an immanent or intrinsic teleology according to which the goal or end is intrinsic to the being itself, which thus has an internal, end-directed principle of change. The being need not be conscious or aware of this intrinsic end.’Footnote ⁵⁹ In turn, Alison Simmons's view, as summarized by Detlefsen, is that Descartes ‘does take the mind–body composite to be a teleological system’, thereby adopting a ‘natural teleology’ in the medieval sense of ‘the attribution of ends to non-rational bodies and their parts’; this version is ‘perfectly legitimate in Descartes’ philosophy’, covering situations such as digestion and other physiological processes.Footnote ⁶⁰ Therefore, for both scholars, teleology sits rather uncomfortably in Descartes's physiology, its role being reduced to supporting the end goal of the survival of the mind–body composite.

However, Simmons and Detlefsen, as well as Gaukroger, argue that teleology for Descartes is not to be found in human bodies as such, isolated from the mind – that is, in any living bodies in general when considered apart from the mind, ‘because such systems are composed merely of matter which is moving in accordance with descriptive laws of nature’.Footnote ⁶¹

My findings show that Descartes consistently allows for a type of natural teleology in living human bodies, not taken as wholes but, specifically, in their partial processes which make up the physiological functioning, and in particular in digestion which is conceived of as a teleological process. I address this with regard to the gradual refinement of chyle up to the stage of blood formation, discussed in the context of an analogy with fermentation in the natural world.

Descartes explains in the Cogitationes, ‘Quod subtilius est, celerius movetur’ (‘what is more subtle, moves at greater speed’).Footnote ⁶² Yet not only speed, but also the ensuing direction of particles of matter, turns out to be a function of their dimensions, since direction is a result of the interaction between the geometry of the particles themselves and of the vessels through which they flow. That direction is necessarily determined already through geometry need not speak for goal-directedness, since the causation goes the other way around; however, we know that these movements of chyle and blood typically only go in one direction of the digestive ‘workflow’, with the culmination consisting in the formation of new blood and spirits. So, too, for Descartes in L'homme:

The image used as an analogy, the maturation of grapes during vinification, contains its own unidirectionality and also teleology: the grapes turn into wine (and never the other way around); maturation is usually – and certainly so in the case of winemaking – understood as a goal-driven process, where the goal is the end product. This would parallel the Aristotelian final cause, described as ‘that for the sake of which the thing is formed – since that is the logos of the thing – its rational ground, and the logos is always the beginning for products of Nature as well as for those of Art.’Footnote ⁶⁴ This seems quite applicable to spontaneous fermentations (found both in nature and in the human body, as accords with a seventeenth-century understanding of physiology).Footnote ⁶⁵

But the Cartesian quote above may be an even better illustration of another Aristotelian concept: that of necessity. To say that the geometry of the hepatic pores is arranged in such a way as to direct the fluid towards its ameliorative transformation is akin to what Aristotle would have called the causation by ‘simple’ necessity, present in natural objects. This he called ‘a third mode of Necessity’, ‘seen in the things that pass through a process of formation’.Footnote ⁶⁶ ‘Necessity’ was an extension, a side note on his system of causes in the natural world; whereas the ‘final cause’ carries most weight, some ‘things come to be by necessity, and so their causes must be referred back to the matter and to the source which initiated their movement’ (i.e. the material and the motive cause).Footnote ⁶⁷ Aristotle specifically used a physiological process, respiration, to illustrate how necessity works:

We must note that the causation described above may go both ways: sometimes the final cause determines the shape of things (because an axe must chop wood, it will be made of hard material), but other times the very nature of things determines the course – such as in Aristotle's biological example.

For Descartes this applies similarly: (a) if formation of blood has to be the end goal of digestion, then the chyle has to undergo processes of refinement; if the fluid entering the liver is to be subtilized, the pores of the liver need to be shaped a certain way; (b) other times, because matter is in a certain way, it is led towards certain ends: due to matter being subtle and hot, it is led towards fermentation, which in turn causes the rarefaction and the formation of blood. Thus Descartes's account of physiological fermentation turns out to parallel some of Aristotle's ideas about causation and necessity.

The processes undergone by the crushed grapes, as well as by fresh, moist hay, are indeed both instances of what we would also call fermentation today. Descartes, as well as many of his contemporaries, used analogies like those referring to fermentations (maturations) to describe physiological phenomena. In the original texts, I have found a stronger reference (than in the available translations) to practices of vintage that were well known to his contemporaries, such as when the grapes were left to ferment together with the grapevine shoot, the rachis. The expression used by Descartes is sur la rape, or, as it appears in L'homme, raspe – identifiable as râpe or rafle. This is a technique which results in a vendange entière, giving the wine a toastier aroma; while it is rather rare today, as most vintners remove all the stalks mechanically in a process called éraflage, it may have been more popular in Descartes's time.Footnote ⁶⁹ By referring to the grapes maturing on the rachis, Descartes underlines the natural process, with as little human intervention as possible (the step of mechanically removing the stems being omitted). The analogy with an almost naturally occurring process enables Descartes to argue that the tendency to ferment is inscribed in the blood itself, the fermentational fire in the heart paralleling the one that emerges naturally when grapes are left to mature on their own.

In this manner, a hint of teleology from the natural world has found its way into Descartes's description. Traditionally, too, the formation of blood has been the end product of a series of fermentations, the last one usually being located in the liver (just as Descartes places it), and it is undisputed that the formation of new blood was a culmination of these processes of gradual refinement. It becomes quite difficult, if not impossible, for Descartes to circumvent teleology here altogether with regard to the human body, because the directionality is transferred from the fermentation model. In fact, Detlefsen also admits that Descartes might have introduced teleological function for the body-in-isolation, in his biological works such as Treatise on Man:

Yet she does not confer on that sort of teleology a full status, suggesting instead that what Descartes might rely on is ‘quasi-teleological accounts of biological processes in all living things’.Footnote ⁷¹ A similar stance is found in Gaukroger, who writes of an ‘apparent goal-directedness of certain physiological processes’ (which nevertheless ‘is the most serious challenge to a mechanist physiology’).Footnote ⁷²

When Descartes writes that the chyle is ‘refined and transformed’, while this still happens by mechanical means, it remains a process with a clear direction.Footnote ⁷³ Accordingly, I would like to nuance Thomas Fuchs's claim that fermentation is not an ennobling process for Descartes.Footnote ⁷⁴ It is difficult to argue that the subtilized and elaborated products of physiological fermentations, such as blood and spirits, would not be superior to masticated food, if we consider a system of causation in which the final cause has ontological priority. I thus argue that Descartes's project of erasing teleology from fermentation was ultimately only moderately successful, and part of the reason may be not just his reliance on analogies with fermentation processes from the natural world, but his stance that the processes happening in the human body are actually fermentative.

Descartes's ontological commitment to the fermentation analogies

Descartes's fermentation analogies, I argue, carry epistemological weight due to the philosopher's ontological commitment.Footnote ⁷⁵ In other words, Descartes indeed believes that there are actual fermentations going on in the human body. The fact that these fermentations slightly differ amongst themselves is irrelevant for the issue of their classification in the same group. One could say, with G.E.R. Lloyd, that they belong to the acceptable semantic stretch of the term.Footnote ⁷⁶ Today we also conceptualize ‘fermentation’ as having different versions: the fermentation of milk, for instance, is a lactic fermentation, and we know that it is produced by lactic bacteria – different from the fermentation of beer or of bread, which is caused by yeast and is classified as an alcoholic fermentation. Yet we still say that the fermentation of milk is like the fermentation of beer, and what we mean is that while these processes are not identical, we have agreed to categorize both of them as valid ‘fermentations’. In a similar way, Descartes and his contemporaries wrote about ‘fermentations’ in digestion and the rest of physiology, presuming that what happens in the human body during digestion would belong to the same class of natural phenomena to which the observable fermentations of wine or hay were assigned. In that sense, Descartes's analogies of physiological processes and natural phenomena were more than a simple figure of speech. They belonged to a reasoning that could qualify as transdiction, a common early modern device of philosophical-scientific inquiry. As Margaret Osler explains, transdiction is ‘a form of inference by which one reasons from the observed properties of macroscopic bodies to the unobservable properties of their microscopic components’. Transdiction was, according to Osler, ‘intrinsic to the mechanical philosophy’; however, it was not without its perils.Footnote ⁷⁷ Its ‘problematic character’, I would argue, stems from a rather simple fact: one does not, and Descartes did not, possess an exhaustive knowledge of those visible macroscopic phenomena that one deploys to explain the invisible. In early modern Europe, fermentation is a common phenomenon, easily observable in daily life and hence easily lending itself as an analogy for physiological processes. But the efficient cause of fermentation remained unknown – or rather unverifiable – and therefore the object of speculation; hence the perils of employing it as an analogy.