Kant's philosophical development took place in the German tradition of rationalist metaphysics. In the
Kant argues that while experience is made up entirely of appearances, these appearances are in some way caused by things in themselves. We cannot perceive things in themselves directly; what we perceive must first be interpreted by our senses, and then by our faculties of sensibility and understanding. Our senses and faculties are what make it possible to connect with the world outside our mind, but they also determine the way this connection is made. Though we cannot perceive things in themselves directly, we know they must exist because there must be some cause behind the appearances we meet with in experience. The existence of things in themselves is crucial to Kant's philosophy, but he insists that we cannot know anything about them.