"But when a monad has organs that are adjusted in such a way that, through them, there is contrast and distinction among the impressions they receive, and consequently contrast and distinction in the perceptions that represent them [in the monads] (as, for example, when the rays of light are concentrated and act with greater force because of the shape of the eye's humors), then this may amount to <i>sensation</i>, that is, to a perception accompanied by <i>memory</i>--a perception of which there remains an echo long anough to make itself heard on occasion."
Principles of Nature and Grace Based on Reason
Leibniz, G. W. <u>Philosophical Essays</u>. Ed. and Trans. Roger Ariew and Daniel Garber. Indianapolis and Cambridge: Hackett Publishing Co., 1989.