The following is taken from an extended work on Aristotle's theory of material substances that is forthcoming from Cambridge University Press. The course of developments leading up to the present extract is, briefly and roughly, as follows. Part I of the work (§§1–5) deals with the metaphysical theory of substance in the Categories, which is a considerably simplified and scaled-down version of the complete theory as found in various other writings of Aristotle. The most considerable simplifications are (1) the total absence of the notion of matter (8a9–11) is of no theoretical significance), and (2) a certain lack of explicitness as to whether the eidos (in the Categories, usually Englished as ‘species’) of a substantial individual is something essential to that individual, so that anything describable as its ceasing to have or belong in that eidos is tantamount to that individual's ceasing to exist. The question then naturally arises why these features of the full theory, as usually interpreted, should be thus effaced in the Categories version, if, as seems to me highly probable, the Categories is an authentic and mature Aristotelian work; and some suggestions are hazarded along these lines.

Part II (§§6–5) is a ‘first approximation’ of the complete theory of substance, necessarily somewhat attenuated and abstract in the absence of a rich intuitive conception of what a ‘full’ substance actually is – something that the Categories cannot possibly furnish.