Philalethes : The understanding is not unlike a closet from which light is wholly shut out, with only some openings left to let in external visible images; if the images coming into such a dark room would stay there, and lie so orderly as to be found upon occasion, it would very much resemble the understanding of a man.
Theophilus : To increase the resemblance we should have to postulate that there is a screen in this dark room to receive the species, and that it is not uniform but is diversified by folds representing items of innate knowledge; and, what is more, that this screen or membrane, being under tension, has a kind of elasticity or active force, and indeed that it acts (or reacts) in ways which are adapted both to past folds and to new ones coming from impressions from the species. This action would consist in certain vibrations or oscillations, like those we see when a cord under tension is plucked and gives off something of a musical sound. For not only do we receive images and traces in the brain, but we form new ones from them when we bring complex ideas to mind; and so the screen which represents the brain must be active and elastic. This analogy would explain reasonably well what goes on in the brain.
Gottfried Wilhelm Leibniz, New Essays on Human Understanding