The substratum is substance, and this is in one sense the matter (and by matter I mean that which, not being a 'this' actually, is potentially a 'this'), and in another sense the formula or form (which being a 'this' can be separately formulated), and thirdly the complex of matter and form, which alone is generated and destroyed, and is, without qualification, capable of separate existence; for of substances in the sense of formulae some are separable and some are not.