Blackwell, R. Descartes' Laws of Motion

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Blackwell, R. Descartes' Laws of Motion


  • Descartes' Laws of MotionAuthor(s): Richard J. BlackwellReviewed work(s):Source: Isis, Vol. 57, No. 2 (Summer, 1966), pp. 220-234Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: .Accessed: 26/12/2012 15:48

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  • Descartes' Laws of Motion

    By Richard J. Blackwell *

    D1 ESCARTES IS OFTEN said to be the first to have formulated a clear statement of the principle of inertia. As early as 1633 in a work

    entitled Le monde Descartes had worked out a theory of motion which ascribes to each body of the universe the power either to remain at rest or to continue in motion in a straight line. However, this treatise was withheld from publication since it advocated a heliocentric astronomy at the height of the Galileo controversy. Le monde was finally published posthumously in 1664. Despite this postponement, Descartes' views on motion continued to develop and received their first presentation to the public with the appear- ance of his Principles of Philosophy in 1644. This work sidestepped the controversy over Copernicanism with a thinly veiled appeal to the relativity of motion which apparently satisfied Descartes' hesitancy to become involved in the great debate between the astronomers and the theologians of his day. At any rate the Principles of Philosophy presents the same three laws of nature 1 which had first been worked out in Le monde, along with a much fuller explanation of their metaphysical background and physical conse- quences.

    The claim that Descartes was the first to formulate the principle of inertia is based on the first two of his laws of nature, which read as follows: 2

    The First Law of Nature: Each thing, insofar as in it lies, always perseveres in the same state, and when once moved, always continues to move.

    The Second Law of Nature: Every motion in itself is rectilinear, and there- fore things which are moved circularly always tend to recede from the center of the circle which they describe.

    Now compare this with Newton's first law of motion, which is the classic statement of the principle of inertia:

    Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.3

    * Saint Louis University. 2 Principia philosophiae (Amsterdam, 1644), 1The main point of difference is that the II, 37, 39.

    order in which the last two laws are presented 3 Isaac Newton, Mathematical Principles of is reversed in the two works. In this paper Natural Philosophy, Motte translation revised we will follow the sequence given in the Prin- by F. Cajori (Berkeley: Univ. California Press, ciples of Philosophy since this is the more 1962), Vol 1 p. 13 logical ordering of the three laws.

    Isis, 1966, VOL. 57, 2, No. 188. 220

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    Both of these statements of scientific law describe basically the same state of physical affairs. A body at rest will stay at rest in the absence of an external influence; a body in motion will continue always in motion in a straight line in the absence of an external influence. Let us call this the descriptive meaning of the principle of inertia. At this level Descartes has a legitimate claim to priority in the formulation of the principle of inertia. But the meaning of a scientific law includes much more than a description of the physical state of affairs. Also involved is an understanding of why the designated physical state of affairs obtains. Let us call this latter factor the theoretical meaning of a scientific law, referring to the entire conceptual apparatus which explains the physical event under discussion. Now it is quite conceivable that two versions of a scientific law may agree at the level of descriptive meaning but not at the level of theoretical meaning. In this case the two versions are not really " the same law." For example, Kepler and Newton would agree in regard to the actual paths described by the planets around the sun, but not in regard to why this is the case. When Kepler's laws were subsumed within Newtonian physics, they acquired a new theoretical meaning.4

    The purpose of this paper is to determine the theoretical meaning of Descartes' version of the principle of inertia. Does this differ from the theo- retical meaning of Newton's first law? To answer this question it will be necessary to investigate the conceptual framework within which Descartes sets his discussion of the problem. Fortunately Descartes is rather explicit on this issue. He carefully worked out the metaphysical preliminaries which he thought were needed as a base for his laws of nature. He also developed a complicated theory of collision (his seven rules of impact), which illustrates the use of his laws at the concrete level. From this data we should be able to establish the theoretical meaning of his version of the principle of inertia.

    Our task is complicated, however, by the fact that his rules of impact are almost completely erroneous. (This is perhaps already an indication that the theoretical meaning of his inertial principle is considerably different from Newton's.) As Alexandre Koyre remarked, the obvious falsity of the Cartesian rules of impact has led the history of science to reject them out of hand without sufficient examination of their systematic role within Cartesian physics.5 It will not be our purpose here to assess the empirical validity of Descartes' rules of impact.6 Our approach rather will be to assume that they are strictly logical consequences of his more basic three laws of nature. If these logical connections can be worked out, then the factors standing behind the rules of impact provide a tool for evaluation of the theoretical meaning of Descartes' version of the principle of inertia. In short, by granting the logical consistency of the laws of nature and the

    4 For a convenient summary of the Keplerian Hermann, 1939), Vol. 3, p. 176. theory of celestial motion and its differences 6 For Tannery's comparison of the Cartesian from the Newtonian view, see J. L. E. Dreyer, and Newtonian analyses of the seven cases A History of Astronomy from Thales to Kepler involved, see (Euvres de Descartes, eds. Charles (New York: Dover, 1953), pp. 393-400. Adam and Paul Tannery (1st ed. Paris: Cerf,

    5 Alexandre Koyre, Etudes Galileennes (Paris: 1897-1910), Vol. 9, pp. 327-330.


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    rules of impact as Descartes presents them, the overall conceptual framework which establishes their theoretical meaning should emerge. With this in mind, then, let us turn to Descartes' first two laws of nature.


    The Cartesian physical universe is generated out of two principles: matter and motion. Once these two factors are given by the divine act of creation, nothing more is needed for the evolution of the endless variety and differ- entiation on display before us in the universe. But there is a very unequal distribution of labor between these two principles. The sole contribution of matter is to provide the spatial extendedness of physical bodies. This is the consequence of Descartes' identification of the essence of material sub- stance with extension.7 Considered in itself, matter is nothing more than three-dimensional extension, or spatial volume. All of the other properties which common sense attributes to material bodies are really due to the motions of the particles of matter among themselves and in relation to man's organs of sense perception. Furthermore, the only type of motion involved here is local motion, defined as the translation of a particle of matter from one set of contiguous neighbors to another.8 Thus spatial extension and local motion generate the universe and constitute its proper explanatory principles. This is indeed a mechanistic philosophy in its fullest sense. Within this mechanism local motion is asked to bear a terrifically heavy burden: everything in the universe other than extension is due to local motion. It remains to be seen whether Descartes' theory of motion can hold up under this strain.

    Granting these two principles, how do they function in the generation of the universe? What governs their interrelations so that a variable yet stable and intelligible universe is produced? Koyre has located the key issue here when he points out that " The supreme law of the Cartesian universe is the law of persistence." 9 Considered in itself, each body of the universe is endowed with the power to maintain its own extension and motion. If left to itself, each body will continue always to persist in its given state of extension and motion. Why is this so? What justifies this law of persistence? If we could address these questions to Descarte