The concept of the thing in itself is one of the most controversial aspects of Kant's philosophy. In Germany, Kant's successors—most notably Hegel—criticized this concept, and advanced a pure form of idealism that did away with things in themselves entirely. It is unclear what sort of relation things in themselves are supposed to bear to appearances if categories such as space, time, and causality do not apply to them. For instance, suppose we see Frank hit John, and then John hit Frank back, we must assume that these appearances are somehow related to things in themselves. But how can there be actions and reactions in things in themselves without the concept of time?

Because he says that they are unknowable, Kant cannot say anything about the nature of things in themselves, but his silence in this regard leaves us with a number of mystifying puzzles. How can appearances in space and time be related to things in themselves outside of space and time? Those who are dissatisfied with Kant generally identify their dissatisfaction with the unanswered questions raised by Kant's doctrine of things in themselves. Idealists generally deny that things in themselves exist, and realists generally assert that categories like space and time have more than just a subjective existence.

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