Deduction, Confirmation, and the Laws of Nature in Descartes's Principia philosophiae

Deduction, Confirmation, and the Laws of Nature in Descartes's Principiaphilosophiae STEVEN M. NADLER THE COMPLEXITIES and ambiguities in Descartes's attempt to establish the three fundamental laws of nature in his Principia philosophiae are well known. Commentators have found it difficult to make coherent sense of Descartes's method here, of what Desmond Clarke calls "an interesting mix of conceptual analysis, empirical corroboration and metaphysical explanation."' Nonetheless , two general positions have emerged in the critical literature regarding this important step in the Cartesian scientific project. On the one hand, the traditional view asserts that for Descartes, the three primary laws of physical motion are deduced (in a logically rigorous sense, on some versions of this view) from God's essence. This deduction fully establishes the laws, confirming them with metaphysical necessity. On the other hand, a somewhat weaker view argues that God's essence only determines what the general features of such laws must be, setting constraints on these principles of the motion of bodies without uniquely determining any particular set. The particular laws themselves are established by recourse to other, "nonmetaphysical" criteria (e.g., explanatory power, simplicity). My intention here is to carve out an intermediate position between these Abbreviationsand references: AT Oeuvres de Descartes, 1~ vols., ed. Charles Adam and Paul Tannery (Paris: J. Vrin, 1974-1982) CSM The Philosophical Writings of Descartes, 2 vols., ed. and trans. John Cottingham, Robert Stoothoffand Dugald Murdoch (Cambridge:CambridgeUniversityPress, 1985) K Descartes: Philosophical Letters, ed. and trans. Anthony Kenny (Oxford: Oxford University Press, 197o) ' DesmondClarke,Descartes"Philosophy ofScience (UniversityPark:PennsylvaniaStateUniversityPress , 198~),97. See alsoBernard Williams,Descartes: The Project of Pure Enquiry (Middlesex: Penguin, 1978),~68. [559] 360 JOURNAL OF THE HISTORY OF PHILOSOPHY 28:3 JULY 199o two extremes. I argue that Descartes, in the Principia, does believe that it is possible to deduce uniquely the laws of nature from God's essence, although this does not necessarily involve a rigorous process of logical entailment. In Le Monde, this connection between God's essence and the laws also serves to confirm these laws as the true ones.' In the Principia, however, the deduction or demonstration functions only as a process whereby God's essence suggests or recommends exclusively certain laws as hypotheses, without confirming them. Confirmation is achieved by means of observation and experiment, as well as other epistemic and nonepistemic criteria. I argue, in other words, that in spite of the almost universal adherence among scholars to the traditional view, even among those who recognize the importance of hypothesis and experiment in Cartesian science,s Descartes's reasoning in the Principia for the laws of nature is quite similar to the modern hypothetico-deductive method. The laws are explanatory hypotheses framed by Descartes on the basis of his knowledge of God's nature; they are then tested (and confirmed) by comparing their deductive consequences with the experimental and other empirical data. 1, The three primary laws governing bodies in motion (and at rest) are presented in the Principia, in Part % articles 37-4~, immediately following a metaphysical discussion of the nature of matter and of motion, and of the primary cause of modon in the universe (God). These laws of nature (regulae sire leges naturae) are described as follows: LI: Each thing, in so far as it is simple and undivided, always remains in the same state, as far as it can, and never changes except as a result of external causes (Principia ~. 37; AT 8-1: 62; CSM 1: 24o-41). " See Le Monde, chap. 7 (AT tl: 36-48). Since his method in LeMonde (1633) does seem to conform to the traditional view, I concentrate here on the later Printipia (1644) and Descartes's mature conception of his scientific project. 3 Even those commentators who argue for a more experimentally oriented Descartes, rejecting the outmoded caricature of a Descartes committed to an endrely a priori, rigorously deductive physical science, insist that at the point where he establishes the laws of nature, Descartes is still working within an a priori framework; he turns to experience and crucial experiments only at a later stage. See Charles Larmore, "Descartes' Empirical Epistemology," in Stephen Gaukroger, ed., Descartes: Philosophy, Mathematics and Physics...