Quote from ACIM Text Chapter 3, Section 3 - Perception versus Knowledge.

We have been emphasizing perception, and have said very little about knowledge as yet. This is because perception must be straightened out before you can know anything. To know is to be certain, and certainty is strength. Perception is temporary. As an attribute of the belief in space and time, it is subject either to fear or to love. Misperceptions produce fear and true perceptions foster love, but neither brings certainty because all perception varies. That is why it is not knowledge. True perception is the basis for knowledge, but knowing is the affirmation of truth and beyond all perceptions.