What is the theme in Kant: A Very Short Introduction by Roger Scruton?

Another theme in the book is phenomena and noumena. Kant attempted to solve the problem of objective knowledge by synthesizing the approaches rationalism and empiricism. He believed that all knowledge of the world came through the senses but that the sense data had to be interpreted by the understanding which applied various concepts and categories to it. Experience, then, was not direct, but rather a representation created by the mind. This results into a distinction between the world as it appears—the phenomenal world—and the world as it really is—the noumenal world. One of the central principles of Kant's transcendental philosophy is that all knowledge is of the phenomenal world; knowledge of the noumenal world is philosophically impossible.