Idealism—Theoretically, the opposite of materialism. But like materialism, idealism is a very broad category of philosophy containing many shades of theory. It is sometimes called mentalism or immaterialism. Idealists believe the universe is the embodiment of a mind. All reality is mental, and matter does not exist. The external world is not physical. Famous idealist philosophers are Berkeley, Hegel, Kant, and Plato. See Materialism, Mind/body problem.