Danish Yearbook of Philosophy, Vol. 43 (2008), 97-118 BODY AND MOTION IN EARLY MODERN PHILOSOPHY OF NATURE: NEWTON AGAINST DESCARTES SUNE FR0LUND Department of Philosophy of Education School of Education, University of Aarhus ABSTRACT, The article addresses the connection of theory of knowledge with physics in Descartes and Newton. The establishment of a geometric concept of motion in Descartes' mechanistic phys­ ics required an epistemic subject with strong constructional powers. Descartes found this in the disembodied, supernatural!. Newton's reintroduction of forces in his kinematics, however, made him ac­ cuse Descartes' interpretation of motion for being non-realistic and contradictory. It also made him attack the mind-body-dualism as the false basis for Cartesian physics. In Newton's physics only an em­ bodied soul could acquire knowledge of the real motion, force and action of (other) natural bodies. The article presents central parts of this debate. 1. Nature - activity or structure? The development of early modem natural science implied a replacement of the older substance metaphysics with a structuralist view of nature. For pre-mod­ em natural philosophy the basic ontological unit was the individual thing, whereas the new perspective focused on the external relations between things. The shift of perspective was implemented through polemics against the idea that natural things are 'substances', i.e. agents having an internal 'nature', an 'essence' or a 'form' that determines their activities or movements. After all, agency was eliminated from substances - things could be dissolved into parti­ cles of specific quantities that were linked in structures made up of geometrical or mathematical relationships (Boudri 2002, 28ff.). The abstraction from autonomous thing to relational structure required a much stronger emphasis on the epistemic activity of the subject. This, again, required an emancipation of the knowing subject from its earthly, bodily or natural restraint. A mind-body dualism was introduced in order to set free the SUNE FR0LUND constructional powers of the subject. The 'Copernican Turn' in astronomy can be considered the paradigm for this liberation of the subject to adopt an exter­ nal viewpoint or even a view from nowhere. The importance of this perspectivism for the development of science cannot be overestimated. Yet the separation of the subject from the body also created epistemological problems that challenged realism in scientific knowledge and gave rise to major disputes between philosophers of nature and scientists. The substance metaphysics of the Aristotelian tradition was sustained by the bodily experiences of the subject. In this tradition the rational soul was embod­ ied, and the inner experience of being a body, even of being a living body, was the primary experience of any physical body and its activities in general. The natural science of this tradition was, consequently, built upon these bodily ex­ periences. This was the case with such concepts from Aristotelian physics as substance, cause and motion, as well as with the Aristotelian concept of nature itself (Spaemann 1987, 20ffV But when monistic ontology was abandoned with the formation of early modern mechanics, the older concepts were either eliminated or - more commonly - reinterpreted to better fit a dualistic ontolo­ gy. This was in particular the case with dynamical concepts like movement, force, power, cause, impulse, striving, endeavour, etc. As could be expected, the mixture of old and new interpretations gave rise to a large number of inco­ herent theories and many disputes, especially in the seventeenth and the eigh­ teenth centuries. In this article I shall present one of the lesser known disputes on the connec­ tion of mechanics with bodily experience. We know of the pivotal role played by the separation of mind from body in the epistemology and physics of Rene Descartes (1596-1650), because Descartes published the philosophical back­ ground for his natural science. In contrast, little is known of Isaac Newton's (1642-1727) philosophy of nature, since so little of it was published by the author. This, however, should not make us believe that Newton was indifferent to philosophical matters or to Cartesian philosophy in particular. In fact, the study of Descartes' physics and philosophy was an important step for Newton in developing his own physics. This article will focus on Newton's rejection of Cartesian dualism and on his arguments for the importance of embodied knowl­ edge for the true understanding of nature. I shall start by outlining the tradi­ tional picture of Newton. .