Hume's skepticism arises when he asks how we perceive causal connections between events. Reason alone cannot tell us about connections between things in the world, and experience alone cannot infer universal generalizations such as "every event has a cause." Hume concludes that in fact we have no rationally justified knowledge of cause and effect. He suggests instead that our concept of causation is justified only by the habit of seeing certain events follow from certain other events.

Kant agrees that we cannot discover the concept of cause and effect either in experience or by means of reason. However, he does not conclude with Hume that this concept is merely a result of habit or custom.

Rather, he suggests, causation is an a priori concept of the understanding applied to appearances. We can know nothing about things in themselves; we can know only how they appear to us in the form given to them by our faculties of sensibility and understanding. The concept of cause and effect is not to be found in these appearances; rather, it is part of the form given to them by the understanding. Causation is not a "thing" that we can discover, either by means of reason or experience. Causation is a form given to experience that makes it intelligible to us. Hume asked how we can derive pure concepts (such as causation) from experience, and answered that we cannot. Kant agrees: we cannot derive pure concepts from experience; rather, we derive experience from these pure concepts.

Pure concepts of the understanding make experience legible, so to speak, but cannot tell us anything about things in themselves. Because pure concepts, as well as our pure intuitions of space and time, are a priori and therefore necessary, we are tempted to think that they can give us knowledge beyond that which we find in experience. However, our pure concepts and pure intuitions provide only form, and no content. They help us make connections between appearances, and as such, they deal only on the level of appearances. They cannot tell us anything about the things in themselves behind these appearances.

Nature, understood as the totality of all our sensations, is possible—as we saw in the first part—by means of our pure intuitions of space and time. Nature, understood as the totality of experience as understood and connected by laws, is possible—as we saw in this part— by means of our pure concepts of the understanding. We cannot go farther and ask how the faculties that give us our pure intuitions and pure concepts are possible, because it is precisely these faculties that help us make sense of experience. We have no further faculty that would help us understand what is behind these faculties.

Sensations themselves teach us nothing about the connections between them or the laws that govern them: these are all provided by our faculties of sensibility and understanding. These faculties, then, are what make nature itself possible, insofar as nature is our intelligible experience. Whatever laws or universality we find in experience comes not from the sensations themselves but from the form given to them by our faculties. Thus, Kant concludes: "the understanding does not derive its laws (a priori) from, but prescribes them to, nature."

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