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Home , Arationality, Mechanical philosophy, Paradigm, Scientific Revolution

Descartes and the Scientific : Some Kuhnian Reflections

Daniel Garber Princeton

Important to Kuhn’s account of scientiªc change is the that when are in competition with one another, there is a curious breakdown of rational and communication between adherents of competing pro- grams. He attributed this to the that competing paradigms are incom- mensurable. The incommensurability thesis centrally involves the claim that there is a deep conceptual gap between competing paradigms in . In this paper I argue that in one important case of competing paradigms, the Aristo- telian of the properties of bodies in terms of and form as opposed to the Cartesian mechanist , where the properties of bodies are explained on the model of , there was no such conceptual gap: the notion of a was as fully intelligible on the Aristotelian paradigm as it was on the Cartesian. But this does not mean that the debate between the two sides was conducted on purely rational terms. Rational argument breaks down not because of Kuhnian incommensurability, I argue, but because of other cultural factors separating the two camps. ’s Structure of Scientiªc (Kuhn 1970) was, from the beginning, closely connected with a particular scientiªc revolution, the Scientiªc Revolution of the sixteenth and seventeenth centuries. Kuhn’s seminal book followed closely on the heels of his (Kuhn 1957), and the astronomical revolution of the period served as one of the central examples of what Kuhn meant by a scientiªc revolution. In this paper, I would like to return to the period and reassess the extent to which the Scientiªc Revolution really ªts Kuhn’s paradigm, as it were. However, I don’t want to deal with the astronomical issues that are at the Earlier versions of this paper were given at Virginia Tech, University of Wyoming, and the University of California, Berkeley. I would like to thank audiences there for their helpful comments on earlier drafts.

Perspectives on Science 2001, vol. 9, no. 4 ©2002 by The Massachusetts Institute of

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center of Kuhn’s account. Rather, I would like to turn to a somewhat dif- ferent issue, the transition from the Aristotelian natural of substantial form and prime matter and to the of Descartes and his contemporaries—another central aspect of the Scientiªc Revolution. On Kuhn’s account, of course, a scientiªc revolution occurs when one paradigm is replaced by another. Kuhn noticed, though, that when para- digms are in competition with one another for the and of the scientiªc community, there is a curious breakdown of rational argument and communication between adherents of competing programs. Kuhn writes: As in political revolutions, so in paradigm choice—there is no stan- dard higher than the assent of the relevant community. To discover how scientiªc revolutions are effected, we shall therefore have to ex- amine not only the impact of and of , but also the tech- niques of persuasive argumentation...(Kuhn 1970, p. 94). Similarly: Like the choice between competing political , that be- tween competing paradigms proves to be a choice between incom- patible modes of community life (Kuhn 1970, p. 94). Similarly, Kuhn talks about the change from one paradigm to another in terms of or the aspect shift we experience when seeing the duck or the rabbit in the familiar ªgure. One paradigm is replaced by another . . . not by deliberation and interpretation, but by a relatively sudden and unstructured like the gestalt switch. Scientists then often speak of the “scales falling from the eyes” or of the “lightning ºash” that “inundates” a previously obscure puzzle, en- abling its components to be seen in a new way that for the ªrst permits its solution (Kuhn 1970, p. 122). As Kuhn puts it in a memorable phrase, “the proponents of competing paradigms practice their trades in different worlds” (Kuhn 1970, p. 150). Central to Kuhn’s program is a particular explanation of this phenome- non often observed in scientiªc change: the incommensurability thesis. According to the incommensurability thesis, there is a genuine disconnect between one paradigm and another. As he characterizes the situation in Structure, a paradigm embodies a number of different elements of scientiªc practice. He writes:

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Paradigms provide scientists not only with a map but also with some of the directions essential for map-making. In learning a para- digm the scientist acquires theory, methods, and standards to- gether, usually in an inextricable mixture (Kuhn 1970, p. 109; cf. pp. 109–10). Changes in any of these components of a paradigm (theory, methods, stan- dards) would appear to render competing scientiªc programs unintelligi- ble to one another. For example, according to the mechanist program, ev- erything has to be explained in terms of size, shape, and . Consequently, the scientist must explain why it is that stay in their or apples fall in terms of the collision between the small bodies that make up the grosser bodies of everyday experience. However, on the Newtonian paradigm, such are not necessary: one can explain such phenomena in terms of an attractive force between every two bodies in the universe. Because of different standards for what counts as a solved scientiªc problem, the two paradigms, the mechanist and the New- tonian, are incommensurable (Kuhn 1970, pp. 104–6). But there is an- other example that points out a feature of competing paradigms that seems more central to Kuhn’s conception of incommensurability. Kuhn points out that Newtonian mechanics and Einsteinian mechanics don’t even share a common vocabulary: the notions of mass, velocity and posi- tion that are at the core of ’s world are very different from the cor- responding in Einstein’s theories. For that it is wrong to say that Newton’s mechanics can be derived from Einstein’s, or that New- tonian mechanics is a special case of Einstein’s (Kuhn 1970, pp. 101–2). In this way, scientiªc revolutions most often involve “a displacement of the conceptual network through which scientists view the world” (Kuhn 1970, p. 102). In a sense, the adherent of one paradigm literally cannot understand what his opponent is saying: they speak a different language with a different conceptual vocabulary. In his later writings Kuhn focuses more and more on the conceptual incompatibilities between the theories embedded in competing paradigms. There Kuhn often characterizes incommensurability in terms of differences in the extension of basic predi- cates in the two paradigms.1 While an adherent of one paradigm can learn 1. Rethinking and reformulating the notion of incommensurability is at the center of many of Kuhn’s later papers. See, for example, the essays in part I of Kuhn 2000. In gen- eral, Kuhn came to focus more and more on the relation between theories formulated in competing paradigms, their respective vocabularies and the difªculties in translating be- tween them. See, e.g., the essay, “The Road Since Structure,” in Kuhn 2000, pp. 91–94, where Kuhn characterizes incommensurability in terms of the incomparability of what he calls the lexicons of competing theories.

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to think in another and become “bilingual,” as it were, there is no direct translation from the one to another, so to speak. As I shall understand it, the incommensurability thesis centrally involves the claim that there is a deep conceptual gap between competing paradigms in science, and that as a consequence, one has to go to non-rational means of in order to convince an adherent of one paradigm to adopt another. As attractive as it is as a picture of scientiªc change, Kuhn’s incom- mensurability thesis is, in an important sense, false. I want to argue that in at least one very interesting (and historically very important) case, there is no conceptual gap between competing paradigms at all. None. For an Aristotelian physicist, is ultimately grounded in the irreducible tendencies bodies have to behave one way or another, as embodied in their substantial forms. Some bodies naturally fall, and others naturally rise; some are naturally cold, and others are natu- rally hot; some are naturally dry, and others are naturally wet. For Des- cartes’ mechanical philosophy, though, in nature is to be ex- plained in terms of the size, shape, and motion of the small parts that make up a sensible body. In essence, for the mechanical philosophy, the whole world can be treated as if it were a collection of machines. There could scarcely be a more paradigmatic instance of a Kuhnian scientiªc rev- olution than this, the transition between one scientiªc paradigm and an- other. But, I claim, the case at hand works very differently than Kuhn would have it. I shall try to show that though there were some very signi- ªcant differences between the Aristotelian and the Cartesian, there was one crucial continuity. In particular, I would like to show how the of the machine and the science of mechanics provided a link between the two worlds, one that was fully intelligible to both parties in the dispute. Now, in most of the literature in the , the purpose of undermining Kuhn’s incommensurability thesis is to restore order and to scientiªc debate. If there is no conceptual gap between com- peting paradigms, then will become rational once again. (I have in here the systems of Lakatos, Laudan, and many others in the 1970s, as well as Peter Galison’s more recent conception of a “trading zone”.) But this is not my aim. Kuhn is right, I think, in seeing the break- down of rational argument in many radical paradigm shifts, even if he is sometimes wrong in using the incommensurability thesis to explain why it happens. I would like to end by offering an alternative explanation for the breakdown of rational argument in the case at hand, the Aristotelians vs. the mechanists, one that leads in the direction of an interesting epistemological question.

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Descartes’ Mechanical Philosophy Descartes saw the physical world and its contents as a collection of ma- chines.2 At the end of his Principia, Descartes tells the reader that “I have described this and indeed the whole visible universe as if it were a machine: I have considered only the various shapes and movements of its parts.”3 Later in the Principia he writes: I do not recognize any difference between artifacts and natural bod- ies except that the operations of artifacts are for the most part per- formed by mechanisms which are large enough to be easily perceiv- able by the senses—as indeed must be the case if they are to be capable of manufactured by . The effects pro- duced in nature, by contrast, almost always depend on structures which are so minute that they completely elude our senses (Pr IV 203). Similarly, Descartes—seeking to clarify his position—suggests to an un- known correspondent that, “all the causes of motion in material things are the same as in artiªcial machines” (Descartes to ?, March 1642, Descartes 1964–74, vol. V, p. 546). In his Traité de l’homme of 1633, Descartes even assimilated the to a machine. In the opening pages of that work, Descartes imagines God to have created a very peculiar machine: “I suppose the body to be but a statue or machine made of earth, which God forms with the explicit intention of making it as much as pos- sible like us” (Descartes 1964–74, vol. XI, p. 99). There is no doubt that this machine is supposed to represent the way our bodies actually are, as his account in the later “Description of the Human Body” (1647/8) makes explicit (Descartes 1964–74, vol. XI, p. 226). But what role does mechanics, the science of machines, in Descartes’ thought? There is some ambiguity here, and a bit of uncertainty about the role of the science of mechanics in Descartes’ thought. But in a signiªcant number of places, Descartes fully identiªes the of nature with the laws of mechanics. In the , for example, Descartes talked about the “laws [regles] of mechanics” as being identical to the laws of

2. Some of the material in this and the immediately following sections is condensed from another related paper of mine, Garber 2002a. 3. Descartes, of Philosophy, part IV, section 188 (abbreviated: Pr IV 188). The text can be found in Descartes 1964–74, vol. VIIIb. Because the text is so easily found through the reference to part and section number, I will omit the reference to Descartes 1964–74 in what follows. The translations of Descartes are generally adapted from Des- cartes 1984–91.

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nature.4 Descartes holds that both artiªcial and natural things, both ani- mate and inanimate things must be explained in the same way. Descartes, in fact, goes so far as to say that “my entire is nothing but mechan- ics.”5 And so, to explain anything in Descartes’ physics, we must do a kind of reverse . Descartes writes, again in the Principia: Men who are experienced in dealing with machinery can take a par- ticular machine whose function they know and, but looking at some of its parts, easily form a conjecture about the design of the other parts, which they cannot see. In the same way I have at- tempted to consider the observable effects and parts of natural bod- ies and track down the imperceptible causes and particles which produce them.6 In the mechanical philosophy, mechanics subsumes physics: everything in physics now receives a mechanical explanation, that is to say, everything is explained as if it were a machine. But what exactly does it mean to explain everything as a machine? Be- fore we can address that question, we need to say something about what it means to be a machine. To illuminate that question, let me go back to one of the classic texts in the of mechanics, the Mechanica or Mechanica Problemata, attributed to in the , though now known to be a somewhat later text. In this text, a machine is deªned in terms of its utility, that is to say, in terms of human ends. Mechanics, then, is a sci- ence of things useful to us. The Aristotelian author of the Mechanica writes in the very opening of the treatise: Nature often operates contrary to human interest; for she always follows the same course without deviation, whereas human interest is always changing. When, therefore, we have to do something con-

4. Descartes 1964–74, vol. VI, p. 54:“...lesregles des Mechaniques, qui sont les mesmes que celles de la nature...”. Cf. also “...Mechanicae meae, hoc est Physicae, leges ...”(Descartes to Plempius, 15 Feb 1638, Descartes 1964–74, vol. I, p. 524). 5. Descartes to Debeaune?, 30 April 1639, Descartes 1964–74, vol. II, p. 542. Cf. also Descartes’ remarks to Fromondus: “If my philosophy seems too ‘crass’ for him, because, like mechanics, it considers shapes and sizes and , he is condemning what seems to me its most praiseworthy feature, of which I am particularly proud. I mean that in my kind of philosophy I use no reasoning which is not mathematical and evident, and all my conclusions are conªrmed by true observational data....Soifhedespises my of phi- losophy because it is like mechanics, it is the same to me as if he despised it for being true.” (Descartes to Plempius for Fromondus, 3 Oct 1637, Descartes 1964–74, vol. I, pp. 420–1.) 6. Descartes, Pr IV 203. For a discussion of some of the epistemological implications of this view, see Laudan 1981.

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trary to nature, the difªculty of it causes us perplexity and art has to be called to our aid. The kind of art which helps us in such per- plexities we call Mechanical Skill.7 This is the subject matter of mechanics for Aristotle: the study of artiªcial things, things constructed to do particular tasks for us that nature herself, left unaided, does not do. This conception of the machine is reºected in discussions of mechanics all the way down to Descartes’ day. How, then, can Descartes talk about natural bodies in terms of ma- chines? We can imagine natural things as if they were machines, by imag- ining them as if they were artifacts, things constructed for a purpose. To say that a natural object is like a machine is to say that we can consider it as, or as if it had been made by someone (God, e.g.) for a purpose, and ex- plain it as such. And so, for example, at the beginning of the Traité de l’homme, Descartes imagines God having created the man/machine which is the subject matter of the treatise (Descartes 1964–74, vol. XI, p. 120). This artifact, this machine has its own purpose: to look and behave as much like a real human body as is possible. Since God is the artisan here, of course the resemblance is exact. What is Descartes’ point here in posit- ing such a strange object? As Descartes puts it in a number of places in the treatise, the machine God creates “represents” what happens in the body (Descartes 1964–74, vol. XI, p. 173, e. g). That is to say, we can conjec- ture that in our natural bodies, the phenomena in question are produced in the same way in which they are in the body of the machine God built to imitate us. Elsewhere, the connection between the machine and the natural object is a bit more distant. Let me refer to a passage we examined earlier. Des- cartes writes: Men who are experienced in dealing with machinery can take a par- ticular machine whose function they know and, but looking at some of its parts, easily form a conjecture about the design of the other parts, which they cannot see. In the same way I have at- tempted to consider the observable effects and parts of natural bod- ies and track down the imperceptible causes and particles which produce them (Pr IV 203). Here there is no supposition that God built a machine to mimic nature. Rather the idea is that we can look at nature through the eyes of the me- chanic or engineer, and ask how he would build a machine whose purpose it would be to mimic nature in just the way God’s man/machine mimics

7. Mechanica 847a14f. The translation is taken from Aristotle 1984, vol. 2, p. 1299.

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us. What is the point of this? The point, as I understand it, is to argue that the natural object could be identical to the mechanical substitute that we imagine is being made. For any given natural thing with its particular behaviors, we can substitute a man-made artifact whose purpose it is to mimic nature. Why is this interesting? In establishing a correspondence between the artiªcial machine and the natural body, we can now explain everything that happens in the natural body in exactly the same way in which we ex- plain what happens in the artiªcial machine. That is, we explain things in the natural world as if they had been made by a mechanic who has only parts of different sizes, shapes, and motions to work with when producing his effects. To return to a passage that I already cited earlier, Descartes writes: I do not recognize any difference between artifacts and natural bod- ies except that the operations of artifacts are for the most part per- formed by mechanisms which are large enough to be easily perceiv- able by the senses—as indeed must be the case if they are to be capable of being manufactured by human beings. The effects pro- duced in nature, by contrast, almost always depend on structures which are so minute that they completely elude our senses (Pr IV 203). Nature’s machines here are just like ours, except only smaller. The point of the machine analogy here is just to limit the resources available to us (and to nature) to produce some effect. If the point of the machine analogy is to dictate the terms in which explanations can take place in physics, then the telos drops out, in effect: the telos of the machine, whether the actual ma- chine that the mechanic makes, or the machine we imagine in nature is strictly speaking irrelevant, and plays no role at all in the project of ex- plaining its behavior.

Mechanics before Descartes Descartes’ mechanical philosophy is closely connected to the science of mechanics and the notion of a machine. The science of mechanics, the the- ory and practice of machines, goes back to the ancients, and was particu- larly lively in the years before Descartes’ thought emerges. The main ancient that fed into Renaissance and early modern mechanics were that of pseudo-Aristotle and that of . I have already mentioned the Aristotelian , the tradition of pseudo- Aristotle’s Mechanica. Archimedes’ more mathematical and abstract tradi- tion was also quite important to the development of mechanics. These two somewhat different traditions came together when both texts were redis-

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covered in the Renaissance. Figures such as Benedetti, Tartaglia, Guido- baldo del Monte, and even Galileo combined elements of these two tradi- tions, along with some pieces of others and some original speculations, to produce a genuine renaissance in the science of mechanics.8 Even though they differ in important respects, the Aristotelian and Ar- chimedean traditions in mechanics both center on machines, as does the later tradition to which they give rise. As we discussed earlier, machines are artifacts, things constructed to do particular tasks for us that nature herself, left unaided, does not do. Embedded in this conception of the do- main of mechanics is a certain conception of the natural world, the dis- tinction between the natural and the artiªcial, and a certain related con- ception of the relations between the science of physics (natural philosophy) and the science of mechanics. For sixteenth-century Italian mechanics, as for Aristotle and Archimedes, there was a real division be- tween physics on the one hand, and mechanics on the other. In the termi- nology of the sixteenth and early seventeenth century, mechanics, together with , , and music, were “middle ” (“scientiae me- diae”) or branches of “mixed ” (“mathematica mixta”), branches of mathematics that used physical premises, but which were distinct from the physics from which they borrowed.9 Natural philosophy (physics) treats natural things as they are in themselves, inquiring into their es- sences and the true causes of natural (physical) phenomena. But things in nature don’t always do what we want them to do. Mechanics, on the other hand, treats artiªcial things, natural things as they are conªgured into de- for our beneªt. In this way, it is a supplement to physics proper: it treats at least certain kinds of things not treated in physics, in particular, artiªcial things, machines.10 And similarly, physics contributes to me- chanics. Machines are, of course, made up of natural materials (wood, metal, ropes, etc.) which have their own natural properties. Mechanics must make use of those properties in explaining the behavior of machines. So, for example, heaviness plays a major role in explaining the simple ma- chines (lever, screw, balance, etc.) all of which use human or animal force to overcome the natural effects of heaviness, and show the mechanic how

8. See Drake and Drabkin 1969. This volume contains important selections from these authors in English translation, together with an introductory essay that remains an invalu- able guide to the subject. 9. St. Thomas discusses the term ‘scientiae mediae’ e.g. in his commentary on the De trinitate of Boethius, q5a3ad6; In II Phys. lect. 3, n. 8; Summa Theol. 2–2.9.2, ad 3. ‘Mathematica mixta’ is used by Rudolph Goclenius (Goclenius 1613, p. 672b) and (Bacon 1605, II.8.2). See also Lennox 1986 and Brown 1991. 10. For a discussion of how the art/nature distinction governed later Aristotelian sci- ence, see Dear 1995, pp. 153–161.

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to lift weights in different ways. Treatises on mechanics all assume that one is dealing with heavy bodies, bodies that tend to fall to the center of the earth. The question of the cause of heaviness and free-fall lay outside of the domain of mechanics, and in the domain of physics. It was one of the premises that the mechanic could borrow from the distinct science of physics. Now, when you talked about physics in the sixteenth and early seven- teenth century, it was very difªcult to avoid Aristotle. Especially interest- ing in this connection is the variety of attitudes toward Aristotle’s physics among writers on mechanics in the period. Some practitioners accept Aris- totle quite willingly, but others reject him quite completely. In this way, even though mechanics is connected with the science of physics, it is not necessarily connected with one physics or another. Speciªc approaches to projectile motion, Benedetti’s for example, do entail giving up elements of Aristotle’s physics of motion, to be sure. But it is fair to say, that mechan- ics as practiced in these ªgures is often combined with a commitment to elements of an , as in the case of Guidobaldo. Though the mechanical philosophy will later itself against Aristotelian natural philosophy, the science of mechanics, as practiced in sixteenth-century is not, in and of itself, anti-Aristotelian. It was by no means clear that doing mechanics entailed one particular natural philosophy over another; in par- ticular it didn’t entail being anti-Aristotelian.11 Let me make one last comment about pre-Cartesian mechanics. Al- though mechanics is deªned at least in part in terms of the Aristotelian distinction between the artiªcial and the natural, in practice “mechanical” reasoning was sometimes applied to natural things too. In the pseudo- Aristotelian Mechanica, for example, the author uses the tools of mechanics to explain a variety of natural phenomena, such as why pebbles on beaches are rounded (§ 15), or why when people stand from a sitting position they make an acute angle between the thigh and the lower leg (§ 30). In this way, the boundary between the natural and the artiªcial was somewhat ºexible. Even from the point of view of the Aristotelian, there are some natural phenomena that are to be explained not in terms of forms, quali- ties, and prime matter, but in terms of the size, shape, and motion of its parts, as if they were machines.

Incommensurability Revisited At this point I want to return to the larger questions raised at the begin- ning of my talk about paradigm shifts and incommensurability. To what 11. On Benedetti’s anti-, see Koyré 1967. On Guidobaldo and Aris- totle, see Meli 1992.

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extent does the shift from the Aristotelian world to the Cartesian world involve incommensurable paradigms? Above I noted that for the sixteenth century, mechanics was a branch of mixed mathematics, distinct from yet linked to natural philosophy prop- erly speaking: mechanics was the science of the artiªcial, and physics the science of the natural. Now, if we think of the mechanist revolution as the transition between Aristotelian natural philosophy taken narrowly and the mechanistic natural philosophy, then it does, indeed, look as if we may well be dealing with “a displacement of the conceptual network through which scientists view the world” (Kuhn 1970, p. 102), that is, with in- commensurable paradigms. What can be more different than the Aristote- lian world of and forms and innate tendencies, and the Cartesian world of tiny machines? One would have to work out the details of the case, of course, but it is not implausible that this transition could be made to appear something like a classical case of Kuhnian incommensurability. But there is another way of looking at the transition. The Aristotelian world contained more than just natural philosophy taken narrowly: it con- tained mechanics as well, the science of machines. If we now think of the Aristotelian paradigm as including both natural philosophy, strictly con- sidered, and mechanics, then the transition from the Aristotelian para- digm to the mechanist world looks rather different. In this case, I claim, it is much less plausible to argue that the competing paradigms were genu- inely incommensurable in the robust sense that Kuhn advances. Descartes can be read as extending the scope of mechanics to subsume all of physics: for Descartes, unlike for his predecessors, everything was to be explained in the way in which one explains a machine. In this way, me- chanics ceases to be a mere middle science, and becomes a genuine me- chanical philosophy. In changing its status, though, there are a number of other important changes that follow. Unlike the pre-Cartesian mechanic, which could and generally did assume the heaviness of bodies, Descartes must explain heaviness, and do so in a mechanistic way. Secondly, unlike the earlier mechanic, Descartes’ conception of mechanics is not deªned in terms of a dichotomy between the natural and artiªcial. But however different it may have been from earlier mechanics, I have no doubt that Descartes’ enterprise was fully intelligible to the practitio- ner of pre-Cartesian mechanics, just as pre-Cartesian mechanics was fully intelligible to Descartes. Even if we give up the characterization of me- chanics in terms of artiªcial machines and their purposes, the idea of things that operate through the size, shape, and motion of their parts is as intelligible to the Aristotelian as it is to the Cartesian. Mechanics built machines that operate through wheels, gears, and pulleys long before Des- cartes elevated their trade to the level of a natural philosophy. Though

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Descartes focuses on a different aspect of the machine than others did, the toolbox he had in mind in this connection would have been familiar to any working mechanic. It is true that there is a kind of transformation in the way in which the two groups see the world, even when we take this Aris- totelian paradigm in a broader way. While both Descartes and the Aristo- telian see machines as machines, what the Aristotelian sees as a “natural” object, the Cartesian sees as a machine as well. But, as I pointed out ear- lier, even for the Aristotelian mechanic there are circumstances in which mechanical reasoning is useful for understanding natural things. Descartes, in fact, depended on the fact that his enterprise was intelligi- ble to his Aristotelian reader. For example, Descartes makes the following observation in his Principia Philosophiae: I have used no principles in this treatise which are not accepted by everyone; this philosophy is nothing new but is extremely old and very common. I should also like it to be noted that in attempting to explain the general nature of material things I have not employed any which was not accepted by Aristotle and all other philosophers of every age. So this philosophy is not new, but the oldest and most common of all. I have considered the shapes, motions and sizes of bodies and examined the necessary results of their mutual interac- tion in accordance with the laws of mechanics, which are conªrmed by reliable everyday experience. And who has ever doubted that bodies move and have various sizes and shapes, and that their vari- ous different motions correspond to these differences in size and shape; or who doubts that when bodies collide bigger bodies are di- vided into many smaller ones and change their shapes? (Pr IV 200). There is something a bit disingenuous here. Though the general mode of explanation in terms of size, shape, and motion was certainly accepted by the followers of Aristotelian mechanics in its appropriate domain, they would certainly deny that all natural phenomena can be so explained, just as Descartes would deny that some natural phenomena must be explained in terms of substantial form and prime matter. But again, Descartes is ap- pealing to a common area of intelligibility in order to make his program acceptable to the Aristotelian. Even if the Aristotelian would balk at the claim that everything is explicable mechanically, he cannot claim not to un- derstand it. In this way, machines and the science of mechanics, common both to the Aristotelian paradigm and the Cartesian, provide a clear pivot point between the two worlds, a point of common conception that enables the two to remain intelligible to one another even as they are arguing for radi-

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cally different paradigms. There is, in an important sense, no gap at all: the science of mechanics is the link that connects the one program to the other. I don’t want to generalize what I have found in this particular case to other cases. But perhaps my here might lead one to look at some other cases of supposed Kuhnian revolutionary change with new eyes, and might help us to see some elements that our adherence to the Kuhnian paradigm have caused us to miss.12 I suspect that more often than not, adherents of new paradigms bend over backwards to make their innovations appear conservative and palatable to the adherents of older views.

Arationality revisited I have been arguing that there is no incommensurability, strictly speak- ing, no conceptual gap between the Aristotelian natural philosophy and the new that Descartes introduced. That would seem to open the door to a purely rational explanation of the change from the one to the other. But you don’t have to look very far to ªnd a breakdown of rational argument in the debate at hand and the use of non-rational modes of per- suasion. If my story is right, then why wasn’t it all reason and light? Why wasn’t the transition a matter of simple argument? I would like to suggest that even if he was wrong in his diagnosis of the source of the problem, Kuhn may have been right in putting his ªnger on a very interesting phe- nomenon in scientiªc change. When discussing the transition from the Aristotelian natural philoso- phy to the mechanical philosophy, we have to keep in mind how deeply entrenched Aristotelian natural philosophy was in European society. When it was (re)introduced into the Latin West in the twelfth and thir- teenth centuries, Aristotelianism was controversial and opposed by many, indeed, condemned and forbidden in such important seats of learning as Paris. However, by the sixteenth century, it had become central to the uni- versity system, at the foundation of the arts curriculum and fundamental to the higher faculties as well. Because of this, in the early seventeenth century it was the common intellectual coin of the realm, what every edu- cated person could be expected to know. Furthermore, Aristotelian natural philosophy had become closely associated with orthodox . For ex- ample, the Catholic of the Eucharist and transubstantiation had been interpreted in terms of the Aristotelian physics of matter, form, and

12. This, of course, is intended to recall Kuhn’s own remarks about how a paradigm can prevent us from seeing things inconsistent with it. See Kuhn 1970, pp. 112ff.

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privation. Because of this, alternative natural , such as - ism, were considered theologically suspicious and on the verge of hereti- cal.13 In this way, the battle between the Aristotelians and their opponents was more than just a battle over one doctrine or another: it was a kind of proxy for a deeper social struggle. To oppose Aristotelian philosophy often meant to oppose not only the of Aristotle, but to oppose the in- stitutions in which they were deeply imbedded, including the University and the learned professions that are grounded in the Aristotelian philoso- phy of the schools, including , , and theology. The new non-Aristotelian philosophies were often attacked on those grounds. There was considerable polemic from both sides, exchanges of insults, and in the case of the Aristotelian opponents of new philosophies, legal action and the attempted suppression of opposing points of view.14 In this context, it is interesting to see the way in which the social back- ground insinuates itself even into apparently rational argument. I found the following argument in a pamphlet published by Jean-Baptiste Morin in 1624, responding to a group of opponents who had argued for a version of and against the of prime matter and substantial form. Morin seems to have taken as basic and beyond serious question the Aristotelian view that “matter...andform united are the essence of body as such” (Morin 1624, p. 36). He thus argues that without matter and form, there can be no bodies. And so, he argues, if there is no matter or form, the human being isn’t a body. And this leads to the denial of God. For if man is not a body, then neither is Jesus Christ. So, if there is no matter and form, Christ must have been lying when he declared that “this is my body.” And if God can lie, then there is no God (Morin 1624, pp. 48–9). Thus heresy, blasphemy, and follow “tres- euidemment” from the denial of the Aristotelian doctrine of form and matter. A curious argument, but there it is. It may seem incredible that anyone could accept such an argument, but Morin did. Moreover, others reprinted Morin’s argument, as if it were a serious blow to anyone who wanted to give up Aristotle. Morin’s pamphlet was summarized in the

13. See, e.g., Redondi 1987. Redondi argues that it was the issue of atomism and the Eucharist that was behind the condemnation of Galileo in 1632–33. Scholars generally agree that Redondi is wrong on that score, but he is certainly correct in seeing the theolog- ical suspicions of atomism. 14. I think here especially of one hapless group of anti-Aristotelians in Paris who were shut down and exiled in August 1624 when they attempted to hold a public disputation at which they promised to refute Aristotle. These issues and the anti-Aristotelian disputation of 1624 are treated in greater detail in my essay (Garber 2002b). The most extensive treat- ment of the details of the August 1624 event can be found in Kahn 2001.

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Mercure François, an annual publication that gave the most important news of the year. In that summary, this argument received feature attention (Mercure François 1625, pp. 503–12; esp. p. 512). How could this have happened? It is not out of the question that Morin was simply not very smart. That is likely to be true. But there is more going on than that. Rarely, if ever, is an argument so powerful that it literally compels assent. People as- sent or fail to assent to in part because of their strength or weakness, in part because of the way in which they interact with their own underlying assumptions and beliefs, and in part because of the non- intellectual stakes there might be for accepting or rejecting a . For Morin and his conservative contemporaries, to give up Aristotle meant for them major changes in their beliefs, indeed, in a whole way of life. For those who taught in the , it meant having to throw away their carefully prepared lecture notes and start all over again, learning some- thing new, lecturing on something new, particularly troubling because it wasn’t clear just what was to replace the Aristotelian philosophy that had endured for so long. For those who practiced the learned professions grounded in the Aristotelianism of the universities it meant having to re- think the basis on which they practiced their crafts. For those used to the accommodation between natural philosophy and theology that had been worked out over centuries, it meant starting from scratch and the possibil- ity of real conºict between and science. Given what it would mean to adopt one of the new anti-Aristotelian philosophies then being advanced, the individual departures from Aristotelian doctrine did not, and perhaps could not add up to good for rejecting Aristotle. At least not at that moment. In this way, one might say, the baggage, ideo- logical and, in a sense, very practical that comes with opposing Aristotle led Morin and his conservative contemporaries to overestimate the strength of the Aristotelian philosophy, and blinded them from taking op- posing points of view as real challenges. (One could say the same about their opponents, who placed much too much conªdence in their own ar- guments for various alternatives to Aristotelianism, overestimating their own strengths, and underestimating the strength of the Aristotelian phi- losophy.) The cultural associations between Aristotelianism and orthodoxy, anti-Aristotelianism and heterodoxy lowered the bar for some arguments, and raised the bar for others, made some arguments salient, and blinded reason to others, at least from the point of view of participants like Morin. If we leave aside the cultural context, and look just at the exchange of reasonings, the case at hand looks very, very strange. But when we under-

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stand the way in which the cultural associations of Aristotelianism and anti-Aristotelianism alter the plausibility of reason and argument in this situation, it becomes considerably more intelligible. Some of Kuhn’s descriptions of paradigm change seem remarkably apt descriptions of the situation here too. “As in political revolutions, so in paradigm choice—there is no standard higher than the assent of the rele- vant community” (Kuhn 1970, p. 94). Like the choice between competing political institutions, that between competing paradigms proves to be a choice between incompatible modes of community life (Kuhn 1970, p. 94). “The proponents of competing paradigms practice their trades in different worlds.”15 However, I would hold, the differing worlds in ques- tion are not the conceptually different worlds of incommensurable para- digms, but the different worlds of different social and political groups. There is often a gulf between competing paradigms, as Kuhn astutely ob- served. But sometimes the gap isn’t always a purely intellectual gap. What I am saying may sound a bit like radical social constructivism, or the view that argument is just a cover for social and political negotiation. That isn’t what I mean at all. The phenomenon to which I am calling at- tention here is really quite familiar from other domains. The fact that we want to believe in the faithfulness of a beloved spouse, or the honesty of a friend does, and perhaps even should raise the standards for that would shake our in them. Another domain in which this phenome- non is common is politics. There may be some people whose politics is completely and dispassionately rational. But for most of us, our political commitments are much more complicated than that. Whatever they are, they deeply affect the way in which we regard arguments that impinge upon political commitments we have. It is, presumably, an objective fact whether or not a given person accepted a bribe, say. But if that person is a political enemy, we may well be much more easily convinced of his than if he were an ally. Similarly, social scientiªc studies of questions con- cerning pornography, welfare, homosexuality, violence in the media, and dozens of other issues are received very differently depending on the polit- ical commitments of the readers. But it isn’t just a matter of ideological assumptions coloring the plausibility of a claim: it is also a question of practical consequences, as I noted earlier. When I was in graduate school, there was an itinerant philosopher making the rounds of the major philos- ophy departments on the East Coast, with a crazy argument for the exis-

15. Kuhn 1970, p. 150. In his presidential address to the PSA in 1990, “The Road Since Structure,” the notion of incommensurability, though rethought and reformulated, re- mains central.

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tence of God based on the “astounding” patterns and regularities one could ªnd on a randomly chosen page of the New York telephone direc- tory (a copy of which he always carried with him). He offered $1,000 to anyone who could give him a refutation of his argument. (Robert Nozick, as I remember, spent more than an hour with him.) One can imagine how high his standards were for a successful refutation! (Needless to say, Nozick didn’t get the money.) What this suggests to me is not that we should give up on reason, and abandon ourselves to , non-rational persuasion, and purely socio- logical analysis of conºict and intellectual change. Rather, I think, what we need is an of , an account of how arguments are weighed in the context of the complicated situations of real debate and ar- gument. In this, perhaps, we shall ªnd at least part of the answer for the striking break-down of communication that is characteristic of Kuhnian paradigm shifts.

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