The one wholly true thought one can hold about the past is that it is not here. To think about it at all is therefore to think about illusions. Very few have realized what is actually entailed in picturing the past or in anticipating the future. The mind is actually blank when it does this, because it is not really thinking about anything.
The purpose of the exercises for today is to begin to train your mind to recognize when it is not really thinking at all. While thoughtless ideas preoccupy your mind, the truth is blocked. Recognizing that your mind has been merely blank, rather than believing that it is filled with real ideas, is the first step to opening the way to vision.
(ACIM. LESSON 8)