Motion, Action, and Tendency in Descartes' Physics

Motion, Action, and Tendency in Descartes' Physics THOMAS L. PRENDERGAST THE PRESENCEOF ACTIONor tendency to move in Descartes' physics has always seemed to be anomalous in the face of his reiterated claim that the whole corporeal world can be explained in terms of matter and motion. One would initially assume that motion is actual rectilinear translation. However, due to certain systematic pressures within Cartesian physics which will be discussed below, the fundamental concept is not actual motion but tendency to move. This paper will sketch the background of the theory of motion showing the necessity of introducing tendency, indicate how tendency functions in the explanation of light as action, and, finally, show the interrelations of motion, tendency and action. I. BACKGROUNDOF THE THEORY OF MOTION Descartes' view of motion is governed to a great extent by his geometrization of the corporeal world. Matter is reduced to rectilinear translation. In the reduction of motion, we can understand Descartes to be rejecting a series of positions on the nature of motion before reaching his definition of motion. First, he rejects the Aristotelian definition, "Motion is the act of a being in potency in so far as it is in potency. ''1 Descartes offers little or no argument supporting his rejection except to say that the definition is obscure and he cannot interpret it.2 In the Meditations, although not speaking of motion, he claims, "Potential being is, strictly speaking, nothing. ''a Of course, if this is true then the definition of motion in which potentiality plays a major role can only be unintelligible to Descartes. His position in the Rules is that motion is something perfectly known by everyone. Not only can the Aristotelian definition not be understood, but also the definition promotes the erroneous view that motion is something composite rather than a simple nature. 4 With the rejection of Aristotelian motion the process character of motion is denied, and all motion or change is reduced to local motion. Descartes makes the point explicitly in the Principles: "I see no more reason to imagine any other [than local motion] in nature. ''5 Second, the common conception of motion must be rejected, namely "the activity by which any body passes from one place to the other. ''e Even if local motion is the only kind admitted, the definition above involves a twofold error. In the first place, as Desx Oeuvres de Descartes, ed. Charles Adam and Paul Tannery, 12 vols. (Pads: Cerf. 18971910 ), XI, 39. AT in future references. If the source of the translation is not indicated, it is mine. 2 Ibid. a Meditations on First Philosophy, Meditaticn !II (AT, VIII, 47). 4 Rules for the Direction of the Mind, Rule XII (AT, X, 426). 5 Principles of Philosophy, II, 24 (AT, VIII, 53). 6 Ibid. [453] 454 HISTORY OF PHILOSOPHY cartes has pointed out in an earlier principle, the notion of place is itself relative, since there is no fixed reference point by which place may be determined. It follows that any change in place will be relative to what point is taken as at rest. The result is that a body may be said to move and not move at the same time, depending on the reference point taken as at rest. Place, therefore cannot be used as part of the definition of motion. In the second place, activity or force cannot be considered as proper to motion itself. Descartes states, "I say it is... not the force or action which transports, in order to show that motion is always in the mobile thing, not in that which moves it.''r The point in this somewhat puzzling text seems to be that motion is to be regarded as a mode or state of the moving body, just as figure is a mode of a figured body and rest a mode of a body in repose. The threefold denial of motion as a process, motion as a change of place, and motion as an activity, is incorporated in Descartes' definition of motion: "We may say in order to assign it a definite nature that it is the translation of a piece of matter or a body from the neighborhood of...