Descartes and the Scientific Revolution: Some Kuhnian Reflections

Descartes and the Scientific Revolution: Some Kuhnian Reflections Daniel Garber Princeton University Important to Kuhn’s account of scientiªc change is the observation that when paradigms are in competition with one another, there is a curious breakdown of rational argument and communication between adherents of competing programs. He attributed this to the fact that competing paradigms are incommensurable. The incommensurability thesis centrally involves the claim that there is a deep conceptual gap between competing paradigms in science. In this paper I argue that in one important case of competing paradigms, the Aristo- telian explanation of the properties of bodies in terms of matter and form as opposed to the Cartesian mechanist paradigm, where the properties of bodies are explained on the model of machines, there was no such conceptual gap: the notion of a machine 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. Thomas Kuhn’s Structure of Scientiªc Revolutions (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 Copernican Revolution (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 Technology 405 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375802 by guest on 25 September 2021 406 Descartes and the Scientific Revolution center of Kuhn’s account. Rather, I would like to turn to a somewhat different issue, the transition from the Aristotelian natural philosophy of substantial form and prime matter and to the mechanical philosophy 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 paradigms are in competition with one another for the hearts and minds 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 nature and of logic, but also the tech- niques of persuasive argumentation...(Kuhn 1970, p. 94). Similarly: Like the choice between competing political institutions, that between 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 religious conversion 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 event 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 time 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: Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375802 by guest on 25 September 2021 Perspectives on Science 407 Paradigms provide scientists not only with a map but also with some of the directions essential for map-making. In learning a paradigm 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, standards) would appear to render competing scientiªc programs unintelligi- ble to one another. For example, according to the mechanist program, everything has to be explained in terms of size, shape, and motion. Consequently, the scientist must explain why it is that planets stay in their trajectories 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 explanations 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 another 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 Newton’s world are very different from the cor- responding concepts in Einstein’s theories. For that reason 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 between 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. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375802 by guest on 25 September 2021 408 Descartes and the Scientific Revolution 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 persuasion 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 incommensurability 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, natural philosophy 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 naturally hot; some are naturally dry, and others are naturally wet. For Des- cartes’ mechanical philosophy, though, everything in nature is to be explained 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 revolution than this, the transition between one scientiªc paradigm and another. 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 idea 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 philosophy of science, the purpose of undermining Kuhn’s incommensurability thesis is to restore order and rationality to scientiªc debate.

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