The understanding prescribes laws to nature

Everything in this unit has been building the machinery (Prolegomena to Any Future Metaphysics §24–Prolegomena to Any Future Metaphysics §35) for one thesis, stated at its climax: the understanding does not draw its laws (a priori) from nature, but prescribes them to nature. Kant sets it out in Prolegomena to Any Future Metaphysics §36 and Prolegomena to Any Future Metaphysics §38. This is the boldest claim in the book.

How can that possibly be true without insane arrogance? Because of what “nature” means. Recall Prolegomena to Any Future Metaphysics §14: nature is the existence of things insofar as it is determined by universal laws — and, we have now seen, an object of experience is possible only under the categories. So the a priori laws (every event has a cause; substance persists) are just the conditions under which any unified, objective experience — any “nature” — can be had at all.

The argument, laid bare: nature-as-we-can-experience-it must conform to these laws because they are what make experience of objects possible in the first place. Anything that failed to fall under the categories could never show up as an object of experience for us — so it could never be part of our nature. The understanding legislates to nature not because it bullies things-in-themselves, but because “nature” already means “the field of possible experience,” and the categories are that field’s constitution.

This is the Copernican revolution applied to natural science. Copernicus, stuck explaining the heavens on the assumption that everything revolves around a stationary observer, tried letting the observer move. Kant makes the same flip: stop assuming the mind must conform to objects, and suppose objects (as experienced) must conform to the mind’s structure. The heavens didn’t change; the standpoint did — and everything fell into place.

And now the prize: this is Kant’s answer to Hume (Prolegomena to Any Future Metaphysics §27). Hume asked where our idea of necessary connection comes from, ransacked experience, and found no impression of it (An Enquiry Concerning Human Understanding VII). He concluded causation is nothing in the objects — only a custom of the mind, a habit of expectation after repeated conjunction.

Here is the turn. Hume was right that necessity is not in the objects, but wrong to conclude it is therefore mere custom. There is a third option he never saw: causality is a pure concept the understanding brings to experience as a condition of any objective experience whatever. So “every event has a cause” is necessary — not because we perceive the necessity in things, and not because habit fooled us, but because without it there is no experienced object at all. Necessity is real; its source is the understanding.

So the unit’s question is answered. Pure natural science is possible because its a priori laws are the very conditions of experience: the understanding prescribes them to nature, so we can know them in advance, with necessity, of every event that could ever be experienced. “The sun warms the stone” — and Newton’s physics with it — is genuine synthetic a priori knowledge, not habit. Hume’s scepticism is answered; the dogmatists’ overreach is disciplined; science is secured.