Conscious perceptions require two things: a thing to be sensed, and a sense organ to perceive them.
A perception of something seen requires a thing to be seen and an eye to see it, before it can arise. Likewise, a perception of something heard, felt, tasted, or smelled.
Both of these–the sense organs and the things sensed–are more or less in a state of never-ending change. Consequently, the conscious perceptions that arise from them must necessarily also be fickle and subject to change.
(Photo taken on Nam Mountain, Gyeongju, Korea.)
Adapted from the sutra at this link: http://www.accesstoinsight.org/tipitaka/sn/sn35/sn35.093.than.html