From knowledge and perception respectively, two distinct thought systems arise which are opposite in every respect. In the realm of knowledge no thoughts exist apart from God, because God and His Creation share one Will. The world of perception, however, is made by the belief in opposites and separate wills, in perpetual conflict with each other and with God. What perception sees and hears appears to be real because it permits into awareness only what conforms to the wishes of the perceiver. This leads to a world of illusions, a world which needs constant defense precisely because it is not real.
ACIM