From Proclus, On Evils, §31.5-14, trans. Opsomer and Steel, with images.
If matter is evil, we must choose between two alternatives: either to make the Good the cause of evil or to posit two principles of beings. For, indeed, everything that exists in any way whatever must either be a principle of complete beings or stem from a principle. Now, if matter stems from a principle, then matter itself receives its procession into being from the Good. If, on the other hand, matter is a principle, then we must posit two principles of beings which oppose each other, viz. the primary good and the primary evil. But that is impossible. For there can be no two firsts.
If, however, matter is necessary to the universe, and the world, this absolutely great and ‘blessed God’, would not exist in the absence of matter, how can one still refer the nature of evil to matter?
It may seem that Plato himself, too, is drawn, as it were, to both argumentations. For when, in the Timaeus, he calls matter the ‘mother’ and ‘wet-nurse’ of generation and a ‘co-cause’ of the fabrication of the world, it is clear to everyone that he takes matter to be good, since he calls the entire world a ‘blessed God’, and matter a portion of the world. But in the argument of the Eleatic Stranger he refers the cause of ‘the disorder of the universe’ to its substrate, when he says that ‘the world possesses all things good from its composer’, but that the contrary of the good originates in the world ‘from the previous condition’ of the world.
In the Philebus, however, he produces matter itself and the whole nature of the unlimited from the One, and, in general, places the divine cause before the distinction between limit and the unlimited. Thus he will admit that matter is something divine and good because of its participation in and origin from god, and that is never evil. For he asserts that ‘one must look for some other causes of evil and not consider god as its cause’, as is said elsewhere.
Perhaps, then, someone may ask us what our opinion is concerning matter, whether we consider it to be good or evil, and in what respect we may admit either of these options. Let this, then, be our decision: that matter is neither good nor evil.
We should repeat what has been often said about matter, that it is a necessity. Indeed, the nature of good is one thing, that of evil another, and they are contrary to each other. But there is another, a third nature, that is neither simply good nor evil, but necessary. Indeed, evil leads away from the good and flees from its nature; but the necessary is everything it is for the sake of the good, and it has a relation to the good.
If then matter exists for the sake of generation, and if no other nature exists for the sake of matter in such a way that we could call it the goal or the good, then we must say that matter is necessary to generation, that it is not evil and that it is produced by divinity as necessary, and that it is necessary for the forms that are incapable of being established in themselves.