Principles in Newton's Natural Philosophy *
The Idea of Principles in Early Modern Thought
Newton's great work of 'rational mechanics' is supposedly about principles. They're even in the title: Philosophiae naturalis principia mathematica. However, Newton's use of the term 'principle' appears to be unsystematic and opaque. Was 'principle', then, just a 'buzzword' for Newton? Or is there something important underlying his use of the term? I support the latter. While the term 'principle' did not play a central role in Newton's methodology (indeed, I will suggest that hypotheses and
... hypotheses and theories are the important concepts for Newton), once we disambiguate his usage, something systematic arises. What it takes to be a principle, by Newton's lights, is to play a certain kind of role. 2 The only obvious exception to this is J. E. McGuire's brief discussion of principles in (McGuire, 1970). However, the scope of McGuire's conceptual analysis doesn't include Newton's published work. 3 principles in their broader methodological context. Drawing on Newton's epistemic distinction between theories and hypotheses, I argue that Newton's propositionalprinciples are a kind of theory. I then show that what differentiates Newton's principles from other kinds of theories is the function they serve-Newton's principles support his mathematico-experimental method in a crucial way. In section 3, I test my account by turning to a draft manuscript, in which Newton enumerated four 'Principles of Philosophy'. Here, my account of principles is illuminating: of those so-called 'principles', only the ones that (a) are deduced from phenomena and (b) function as premises became principles in Newton's published work. From this discussion, a particular feature of Newton's use of the term 'principle' emerges: labelling or referring to a proposition as a 'principle' tells us about the function, rather than the content, of the proposition. I argue that this highlights a more general lesson, namely, that when we study Newton's methodology, we should emphasise functions and distinctions over content. And so, I digress to study a case that illustrates this broader point. I conclude that Newton's application of the label 'principle' is exactly what we should expect, given his methodology.