25. There are those who lay down four categories and make a fourfold division into Substrates, Qualities, States, and Relative States, and find in these a common Something, and so include everything in one genus.
Against this theory there is much to be urged, but particularly against this posing of a common Something and a single all-embracing genus. This Something, it may be submitted, is unintelligible to themselves, is indefinable, and does not account either for bodies or for the bodiless. Moreover, no room is left for a differentia by which this Something may be distinguished. Besides, this common Something is either existent or non-existent: if existent, it must be one or other of its [four] species;- if non-existent, the existent is classed under the non-existent. But the objections are countless; we must leave them for the present and consider the several heads of the division.
To the first genus are assigned Substrates, including Matter, to which is given a priority over the others; so that what is ranked as the first principle comes under the same head with things which must be posterior to it since it is their principle.
First, then: the prior is made homogeneous with the subsequent. Now this is impossible: in this relation the subsequent owes its existence to the prior, whereas among things belonging to one same genus each must have, essentially, the equality implied by the genus; for the very meaning of genus is to be predicated of the species in respect of their essential character. And that Matter is the basic source of all the rest of things, this school, we may suppose, would hardly deny.
Secondly: since they treat the Substrate as one thing, they do not enumerate the Existents; they look instead for principles of the Existents. There is however a difference between speaking of the actual Existents and of their principles.
If Matter is taken to be the only Existent, and all other things as modifications of Matter, it is not legitimate to set up a single genus to embrace both the Existent and the other things; consistency requires that Being [Substance] be distinguished from its modifications and that these modifications be duly classified.
Even the distinction which this theory makes between Substrates and the rest of things is questionable. The Substrate is [necessarily] one thing and admits of no differentia- except perhaps in so far as it is split up like one mass into its various parts; and yet not even so, since the notion of Being implies continuity: it would be better, therefore, to speak of the Substrate, in the singular.