In the preface to the Prolegomena, Kant asks us to
[P]ause a moment and, disregarding all that has been done [so far in the history of metaphysics], to propose first the preliminary question, “Whether such a thing as metaphysics to be at all possible?”
But before we can ask, with Kant, whether or not it is possible to do metaphysics we need to know what it is that metaphysicians are trying to do in the first place. There are many ways of answering this question but the following definition should be good enough for our purposes: metaphysics is the study, by pure reason alone (i.e. by “mere concepts”), of the fundamental and necessary (or “universal”) structure and nature of reality.
Kant’s question is then “is it possible to know what the world is like (and must be like) just by thinking about it?” And it is important to notice that this question itself can be broken down into two components: a) is it possible to know anything just by thinking about it? and b) is the world one such thing?
Traditionally, both rationalists and empiricists answer ‘yes’ to (a) — namely in regards to mathematics and logic — but only rationalists answer ‘yes’ to (b). This is because empiricists, as we know, believe that all knowledge of the world must be derived from the senses which, unlike reason, treat not of universals or abstracta, but of particulars. That is to say, that the difference between rational objects and sensory objects is a modal difference: reason deals with the realm of the possible and the necessary while empirical science deals with the realm of the actual.
And so, it seems at least prima facie that the objects of thought and the objects of sense are entirely different kinds of things and are thus best delegated to different branches of philosophy for study. However, the solution is not so easy; for empirical science both relies on mathematical and logical reasoning and takes itself to be discovering not only how the objects of nature behave but also how they must behave. That is to say, science, like reason, considers itself warranted to make claims about necessity. A physicist discovers natural laws not merely natural occurences.
But given the difference between the two modally distinct realms of thought and sensation, the emphasis can be shifted in Kant’s question and rephrased also as ‘how is physics possible?’ Irrespective of whom we decide to interrogate, the physicist or the metaphysician, we are still after the same explanation: how to justify universal knowledge claims about the natural world of particulars. Or perhaps more pithily, how come ‘natural law’ is not an oxymoron?
Now, in the Principia, Newton seems to be offering a kind of proto-Kantian ‘Copernican’ inversion-style argument as a means of justifying his physical theory. Essentially, he takes what is agreed by all to be an epistemically justified and foundational system of knowledge, geometry, and grounds it in what has hitherto been taken to be an imperfect and non-foundational system of knowledge, namely, mechanics. Simply put, Newton seems to be saying that geometry would not be possible if the physical world did not have the lawful structure that the Principia is going to describe. And this is presumably because the objects of geometry (e.g. the point, the plane, the line) which we take as given, are in fact rational reconstructions “fetched from without” of fundamental entities and forces within the natural, mechanical world. In a way then, Newton might be suggesting that the atemporal structures of geometry are just ‘frozen’ images of the temporal, three-dimensional forces of physics. I.e. if the world was not Newtonian then Geometry would not be Euclidean!
Is this a fair and fruitful interpretation of Newton’s argument in the beginning of the Principia or is this disastrously off course?