It is one of those distinctions which is obvious, without being sharp or clear. It is obvious, and remains obvious, to every normal mind, although when we come to analyze it, we may not be able to rule a boundary line. It remains obvious, as the distinction between day and night remains obvious, though, when we begin to analyze that distinction, we come up against such refinements as dusk and twilight. There is more than one way of characterizing the difference. Perception is essentially a passive experience, something that happens to us; thinking is an active one, something that we do. Or if you don't like this distinction, because of refinements such as the "intentionality" which some have detected (rightly, I would say) in perception, or on the other hand because of the passivity of that uncontrolled type of thinking called "reverie," then thoughts are something that comes from within; perceptions something that comes from without.