 A long passage in Themistius’ commentary on Aristotle’s De anima (Heinze 1899, 11.18–12.33) seems to stem from Xenocrates’ On Nature (in 11.37–12.1 Themistius says “It is possible to gather all these {things} from the On Nature of Xenocrates”). This is a discussion of a story about the composition of the soul from the formal numbers 1, 2, 3, and 4 (although 1 was not normally considered a number), mentioned in De anima 408b18–27. The motivation for this account of the soul, in both Aristotle and Themistius, is the explanation of how we can know things about the universe: the universe is derivative from those numbers, and so, if the soul is similarly derivative, the soul can know things under the principle that like things are known by like. This cognitive sort of account is contrasted with another motivic type of account, that takes as the primary thing to be explained the fact that the soul can initiate motion.