11. I think, in fact, that Plato had this in mind where he justly speaks of the Images of Real Existents "entering and passing out": these particular words are not used idly: he wishes us to grasp the precise nature of the relation between Matter and the Ideas.
The difficulty on this point is not really that which presented itself to most of our predecessors- how the Ideas enter into Matter- it is rather the mode of their presence in it.
It is in fact strange at sight that Matter should remain itself intact, unaffected by Ideal-forms present within it, especially seeing that these are affected by each other. It is surprising, too, that the entrant Forms should regularly expel preceding shapes and qualities, and that the modification [which cannot touch Matter] should affect what is a compound [of Idea with Matter] and this, again, not a haphazard but precisely where there is need of the incoming or outgoing of some certain Ideal-form, the compound being deficient through the absence of a particular principle whose presence will complete it.
But the reason is that the fundamental nature of Matter can take no increase by anything entering it, and no decrease by any withdrawal: what from the beginning it was, it remains. It is not like those things whose lack is merely that of arrangement and order which can be supplied without change of substance as when we dress or decorate something bare or ugly.
But where the bringing to order must cut through to the very nature, the base original must be transmuted: it can leave ugliness for beauty only by a change of substance. Matter, then, thus brought to order must lose its own nature in the supreme degree unless its baseness is an accidental: if it is base in the sense of being Baseness the Absolute, it could never participate in order, and, if evil in the sense of being Evil the Absolute, it could never participate in good.
We conclude that Matter's participation in Idea is not by way of modification within itself: the process is very different; it is a bare seeming. Perhaps we have here the solution of the difficulty as to how Matter, essentially evil, can be reaching towards The Good: there would be no such participation as would destroy its essential nature. Given this mode of pseudo-participation- in which Matter would, as we say, retain its nature, unchanged, always being what it has essentially been- there is no longer any reason to wonder as to how while essentially evil, it yet participates in Idea: for, by this mode, it does not abandon its own character: participation is the law, but it participates only just so far as its essence allows. Under a mode of participation which allows it to remain on its own footing, its essential nature stands none the less, whatsoever the Idea, within that limit, may communicate to it: it is by no means the less evil for remaining immutably in its own order. If it had authentic participation in The Good and were veritably changed, it would not be essentially evil.
In a word, when we call Matter evil we are right only if we mean that it is not amenable to modification by The Good; but that means simply that it is subject to no modification whatever.