 In the first of our two anchor texts, Aristotle, discussing the ways in which people have answered the question ‘what are the substances?’, ascribes to Plato the view that there are not only perceptible substances but eternal ones of two types: forms and mathematical objects. He then says that Speusippus thought there were even more types of substance; he “started from the One” and adopted principles (archai) for each of his types of substance: “one for numbers, another for magnitudes, then for soul”. So we have at least four layers of beings: numbers, magnitudes, souls, and perceptible beings; the One is Speusippus’ starting-point, but he has different principles for each level of being.