Didascalicon, Book II: the sciences, their parts and their discipline
Chapter VI. On the quaternary of the body
For they assign to the body also its own quaternary. As the monad suits the soul, so the dyad suits the body. Say twice two: it makes 4; say twice 4: it makes 8; say twice 8: it makes 16; say twice 16: it makes 32. Here in the fourth place the same number, namely the binary, from which the multiplication took its beginning, likewise meets you; and the same if you proceed to infinity, it will undoubtedly happen that in the fourth degree the binary always stands out. And this is the quaternary of the body, in which it is given to understand that everything subject to dissolution receives composition, and is itself dissoluble. You see now, plainly enough, I think, how souls degenerate from the intellectibles to the intelligibles: when from the purity of simple understanding, which is darkened by no image of bodies, they descend to the imagination of visible things; and again become more blessed when, gathering themselves from this distraction to the simple fountain of their nature, they are composed as if impressed with the sign of the best figure. There is, then, to say it more plainly, in us the intellectible, which is understanding, and the intelligible, which is imagination. Understanding is the pure and certain knowledge only of the principles of things, that is, hyle, and of incorporeal substances. Imagination is the memory of the senses from the remains of bodies clinging to the mind, having in itself no principle of certain knowledge. Sense is the passion of the soul in the body from qualities befalling from without.