What is pure natural science?

Part I of the Prolegomena asked how pure mathematics is possible, and answered: because space and time are our own a priori forms of intuition. Part II now turns to a second, parallel question — the whole business of this unit. Kant states it flatly: how is pure natural science possible? He opens the inquiry at Prolegomena to Any Future Metaphysics §14.

First, what does “pure” natural science mean? Not the whole of physics — not the boiling point of water or the mass of Jupiter, which you learn only by looking. Kant means the a priori core of physics: the universal, necessary principles presupposed by all the empirical work. A priori = knowable independently of experience, and its mark is necessity and universality (it holds in every case, and couldn’t be otherwise).

His headline examples: “in all changes of the material world, the quantity of matter (substance) remains unaltered” — matter is neither created nor destroyed — and “in all changes, every event has its cause.” No one arrives at these by counting cases. You could not check “every event, everywhere, always.” Yet science treats them as holding with strict necessity, before and beneath any particular measurement.

Second, what is nature? Kant gives it a precise, thin definition. Nature, in the relevant sense, is “the existence of things insofar as it is determined according to universal laws” (Prolegomena to Any Future Metaphysics §14). Not the sum of trees and rocks — but things considered as law-governed. To speak of “nature” at all is already to speak of a lawful order. That is the clue Part II will exploit.

Now the puzzle bites. These principles are synthetic a priori: synthetic because “every event has a cause” adds something not contained in the mere concept of an event (an event is just a beginning-to-be; “caused by something else” is extra content), and a priori because we hold it with a necessity no enumeration of cases could give. So: how can the mind know, in advance of experience, that nature must conform to laws it never read off any object?

So Part II sets itself a task exactly parallel to Part I. There, the possibility of pure mathematics was explained by finding something a priori on the side of sensibility — the forms of intuition, space and time. Here, the possibility of pure natural science will require finding something a priori on the side of the understanding: pure concepts that do for thinking objects what space and time do for sensing them. That parallel is the thread we follow through the rest of the unit.