WE have explained under what conditions combination, contact, and action-passion are attributable to the things which undergo natural change. Further, we have discussed unqualified coming-to-be and passing-away, and explained under what conditions they are predicable, of what subject, and owing to what cause. Similarly, we have also discussed alteration, and explained what altering is and how it differs from coming-to-be and passing-away. But we have still to investigate the so-called elements of bodies.
For the complex substances whose formation and maintenance are due to natural processes all presuppose the perceptible bodies as the condition of their coming-to-be and passing-away: but philosophers disagree in regard to the matter which underlies these perceptible bodies. Some maintain it is single, supposing it to be, e.g. Air or Fire, or an intermediate between these two (but still a body with a separate existence). Others, on the contrary, postulate two or more materials-ascribing to their association and dissociation, or to their alteration, the coming-to-be and passing-away of things. (Some, for instance, postulate Fire and Earth: some add Air, making three: and some, like Empedocles, reckon Water as well, thus postulating four.)
Now we may agree that the primary materials, whose change (whether it be association and dissociation or a process of another kind) results in coming-to-be and passing-away, are rightly described as originative sources, i.e. elements. But (i) those thinkers are in error who postulate, beside the bodies we have mentioned, a single matter-and that corporeal and separable matter. For this body of theirs cannot possibly exist without a perceptible contrariety: this Boundless, which some thinkers identify with the original real, must be either light or heavy, either cold or hot. And (ii) what Plato has written in the Timaeus is not based on any precisely-articulated conception. For he has not stated clearly whether his Omnirecipient exists in separation from the elements; nor does he make any use of it. He says, indeed, that it is a substratum prior to the so-called elements-underlying them, as gold underlies the things that are fashioned of gold. (And yet this comparison, if thus expressed, is itself open to criticism. Things which come-to-be and pass-away cannot be called by the name of the material out of which they have come-to-be: it is only the results of alteration which retain the name of the substratum whose alterations they are. However, he actually says that the truest account is to affirm that each of them is gold.) Nevertheless he carries his analysis of the elements--solids though they are--back to planes, and it is impossible for the Nurse (i.e. the primary matter) to be identical with the planes.
Our own doctrine is that although there is a matter of the perceptible bodies (a matter out of which the so-called elements come-to-be), it has no separate existence, but is always bound up with a contrariety. A more precise account of these presuppositions has been given in another work: we must, however, give a detailed explanation of the primary bodies as well, since they too are similarly derived from the matter. We must reckon as an originative source and as primary the matter which underlies, though it is inseparable from, the contrary qualities: for the hot is not matter for the cold nor the cold for the hot, but the substratum is matter for them both. We therefore have to recognize three originative sources: firstly that which potentially perceptible body, secondly the contrarieties (I mean, e.g. heat and cold), and thirdly Fire, Water, and the like. Only thirdly, however: for these bodies change into one another (they are not immutable as Empedocles and other thinkers assert, since alteration would then have been impossible), whereas the contrarieties do not change.
Nevertheless, even so the question remains: What sorts of contrarieties, and how many of them, are to be accounted originative sources of body? For all the other thinkers assume and use them without explaining why they are these or why they are just so many.