Readers and reviewers generally failed to appreciate the originality of Kant's ideas. Readers interpreted Kant as saying something more familiar to them than what he was actually saying. The idea that rationalist metaphysics, which was the main occupation of philosophers in Germany at the time, could be dismissed entirely was too revolutionary a concept to catch on easily.

One of Kant's main troubles, it seems, was that he was taken for an idealist. Idealism is the doctrine that reality is dependent upon the mind. A common idealistic argument suggests that everything I know about the world I learn through the senses, and so the things I "know" are not external objects and phenomena, but just the report of my senses. My concept of the world, an idealist would argue, is based entirely on sensory images that exist only in my mind, and has at best a dubious connection with the things in themselves that exist in the world.

A famous proponent of this position is George Berkeley, an Irish bishop who argues that esse est percipi—"being is being perceived." He asserts that chairs and tables and the like have no independent existence, that they only exist in the mind of someone who is perceiving them. He evades the odd claim that these things cease to exist when no one is perceiving them by positing the existence of God as a being who is perpetually perceiving everything.

Kant's philosophy is very firm in asserting that we can know only about appearances, and that we can know nothing about things in themselves. This assertion is enough to make Kant an idealist of sorts, but he wants to qualify this title of "idealism." He is not, like Berkeley, saying that only appearances exist: though we can know nothing about things in themselves, they are still a crucial part of his philosophy.

Kant calls his philosophy "transcendental" or "critical" idealism. The "transcendent" world of things in themselves is contrasted with the "immanent" world of appearances. Because he believes that things in themselves exist, his idealism believes in the existence of a "transcendent" world that is behind the world of appearances.

His idealism is "critical" because it is directed toward what we can know, not toward what exists. He is not saying that only appearances exist, but that appearances are all we can know. Kant's critical philosophy questions how we can come to know what we know, so he is an idealist only in saying that we cannot know things in themselves.

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