Keith Hutchison writes…
The core of Kirsten Walsh’s paper is a defence of her proposal that Newton’s De Gravitatione was composed after the publication of the new theory of colours (in 1672-3). Kirsten compares the methodology of the optical writings with that of De Grav. and notes that despite the similarity there are significant differences. Yet the methodology of De Grav. is effectively identical to that of the Principia, so is plausibly interpreted as the one preferred by Newton. So Newton would have displayed this methodology in the optical writings, Kirsten concludes, had De Grav. already been composed.
Though I am (tentatively) happy with Kirsten’s observation that it is uncontroversial to see Newton’s Principia as deploying the methodology of De Grav., part of the reason for this is surely the fact that the discussion of methodology in De Grav. is so brief, and hardly exemplified in the actual science that Newton so fleetingly displays in his text. The little that we find in De Grav. does indeed seem concordant with much that happens in the Principia, but it is easy – too easy – to find agreement between a pair of texts if one of them is vague enough. Given that the identity between the two methodologies is so important to Kirsten’s case, she needs to find some way of sharpening this step of the argument.
She could, for instance, identify far more thoroughly the small differences between the methodology of the optical writings and that of De Grav. If each of these differences could be consistently found in the Principia as well, Kirsten would have a much better case, as long as there were not something about the optical investigations that required the alternative approach. Kirsten notes indeed, that Cohen has suggested that the Principia is primarily a mathematical investigation, but the optical work is overwhelmingly experimental. Cohen seems to be significantly wrong here, for investigations of the context of Newton’s treatment of chromatic aberration show that Newton originally dreamt of creating a mathematical science of colours – until he found that refraction was puzzlingly idiosyncratic, and so unlike the extremely orderly gravitational interaction that provided much of the mathematics of the Principia. But it remains true that the optical work is saturated with experiment, and it could be this that allows an earlier (?) De Grav. to seem more like the Principia.