# Newton’s law–constitutive approach to bodies: A response to Descartes

Published

Book Section

© Cambridge University Press 2012. In his Principia Newton offers us a science of bodies in motion. Such a science has bodies as its subject-matter, but what are these bodies? If Newton's three laws of motion are to say anything, then there must be bodies for them to refer to. I shall label this the ‘problem of bodies’. In this chapter I outline the ‘problem of bodies’ as Newton finds it in Descartes's Principles of Philosophy. I claim that while there is no obvious solution explicit in Descartes's writings, an implicit solution is strongly suggested. I argue that Newton was acutely aware of the problem, and addressed it explicitly by adopting the strategy implicit in Descartes. My claim is that Newton offers a law-constitutive solution to the problem of bodies, according to which the definition of bodies is incomplete prior to the specification of the laws of nature, and completed by those laws of nature. Descartes and the problem of bodies Taken together, Descartes's laws of nature concern the behaviour of ‘bodies’. Here are the laws as he stated them in his Principles of Philosophy (Part II, paragraphs 37, 39, and 40): The first law of nature: that each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move. The second law of nature: that all movement is, of itself, along straight lines; and consequently, bodies which are moving in a circle always tend to move away from the center of the circle which they are describing. The third law: that a body, on coming in contact with a stronger one, loses none of its motion; but that, upon coming in contact with a weaker one, it loses as much as it transfers to that weaker body. The ‘problem of bodies’ is this: what are the ‘bodies’ to which these laws apply? For Descartes, the answer is ‘parts of matter’. Famously, however, this answer masks a difficulty that Descartes never satisfactorily resolved, and which arises as follows.

### Published Date

• January 1, 2012

### Book Title

• Interpreting Newton: Critical Essays

• 13 - 32

### International Standard Book Number 13 (ISBN-13)

• 9780521766180

### Digital Object Identifier (DOI)

• 10.1017/CBO9780511994845.003

• Scopus