DOI: 10.4324/9780415249126-Q075-1
Version: v1,  Published online: 1998
Retrieved April 20, 2024, from

3. Space, time and the laws of motion

Two aspects of the Principia provoked philosophical controversy in the decades following its publication: first, the appeal to absolute space and motion, and second, the insistence on establishing properties of gravity, especially its universality, without appeal to any mechanical hypothesis that could begin to explain how gravity is produced.

The Principia opens with two short sections, ’Definitions’ and ’Axioms, or Laws of Motion’, that have drawn philosophical fire ever since. The distinctions between absolute and relative time, space and motion are drawn in the first of these, following his introduction of the concept of mass and definitions pertaining to quantity of motion and force. For Newton, the distinction between absolute (or true) and relative (or apparent) motion is primary, and the parallel distinctions concerning space and time serve mostly to support this one (see Mechanics, classical; Space §§2–3). Newton was acutely aware of the empirical difficulties raised by such distinctions:

It is certainly very difficult to find out the true motions of individual bodies and actually to differentiate them from the apparent motions, because the parts of that immovable space in which the bodies truly move make no impression on the senses. Nevertheless, the case is not utterly hopeless. For it is possible to draw evidence partly from apparent motions, which are differences between the true motions, and partly from the forces that are the causes and effects of the true motions …. But in what follows, a fuller explanation will be given of how to determine true motions from their causes, effects, and apparent differences, and, conversely, how to determine from motions, whether true or apparent, their causes and effects. For this was the purpose for which I composed the following treatise.

(Newton 1687: Scholium to Definitions)

The reference here is to the laws of motion and their corollaries, which immediately follow this last remark, as well as to the ninety-eight demonstrated Propositions of Book I and the fifty-three of Book II.

Like Descartes, Newton appealed to forces to distinguish true from apparent motions. And, again like Descartes, the true motion of greatest importance to him in the sequel is curvilinear motion, most notably the true motion of the planets that would distinguish between their equivalent relative motions in the Copernican and Tychonic systems. Unlike Descartes, however, Newton refused to offer hypotheses concerning the forces in question (see Descartes, R. §11). He required that the forces be inferred from phenomena with the help of the mathematical principles of Books I and II, many of which licence inferences from observed motions to measures of force. Inconsistencies among the inferred quantities of force or the motions subsequently inferred from them would indicate a failure to be dealing with true motions. But this is an empirical question, to be decided by carrying out the investigation of motions under forces to its fullest extent, insisting on no less than complete agreement between observed and calculated motions. Thus, the successes, and also the limitations, of the appeal to absolute space, time and motion were, for Newton, empirical issues that the long-term development of an exact science of motion would decide, and not something he thought was open to a priori resolution.

Citing this article:
Harper, William L. et al. Space, time and the laws of motion. Newton, Isaac (1642–1727), 1998, doi:10.4324/9780415249126-Q075-1. Routledge Encyclopedia of Philosophy, Taylor and Francis,
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