My father thinks the quote I was looking for is:
Man knows so little about his fellows. In his eyes all men or women act upon what he believes would motivate him if he were mad enough to do what that other man or woman is doing.
~ William Faulkner, Light in August
As a bonus:
You cannot know yet whether what you see is what you are looking at or what you are believing
~ William Faulkner, Absalom, Absalom!: The Corrected Text (1990).
Where did I hear before “we see things not as they are, but as we are?”