The essential top-down contribution of previous knowledge to perception has tempted some
philosophers and psychologists to conclude that the senses do not enable us to know what objects are,
only what they appear to be. Some worry that the gap between appearance and reality cannot be
bridged, as when Kant said that we cannot know things in themselves. Some psychologists writing on
hallucinations have claimed that support for Kant's idealism comes from the brain's capability of
generating illusory perceptions that have no connection with reality. A few micrograms of a drug like
LSD can disengage your brain's perceptual apparatus from the usual sensory inputs and generate
fantastic images that have no correspondence to anything in the world. You do not have to take drugs
to hallucinate, as a similar process takes place every night when you dream. Your brain generates
complex and often compelling sensory experiences that are not directly caused by anything in the
world. Last night I dreamed that I was shopping in a supermarket and bought some delicious bread,
but morning brought the realization that the market and the bread were unreal.