Didascalicon, Book I (chapters 2-13): the origin of the arts and the division of philosophy
Chapter VII. On the three differences of things
Now among things there are some which have neither beginning nor end, and these are called eternal; others which have a beginning but are shut in by no end, and are called perpetual; others which have a beginning and an end, and these are temporal. In the first order we place that in which being and what it is are not different, that is, whose cause and effect are not distinct, which has its subsistence not from another but from itself, as is the Begetter and Artificer of nature alone.
But that in which being is one thing and what it is another, that is, which comes to be from another and has flowed forth into act from a preceding cause so as to begin to be, is nature, which contains the whole world. And it is divided in two. There is a certain kind which, in order to begin to be, proceeds from its primordial causes into act with no mover, by the sole choice of the divine will; and there it stands immutable, without any end or vicissitude. Of this kind are the substances of things which the Greeks call ousiai, and all the bodies of the superlunary world, which for this reason, that they are not changed, have been called divine. The third kind of things is that which has a beginning and an end, and does not come to be of itself, but is the work of nature: those things which arise upon the earth, beneath the lunar globe, by the moving of the artificer fire, which by a certain power descends into sensible things to produce them. Of these, then, it has been said: “Nothing in the world dies, because no essence perishes.” For it is not the essences of these things that pass away, but the forms. And when a form is said to pass away, it is not to be understood as though some existing thing perished utterly and lost its being, but rather that it is varied; as perhaps that things which had been joined are separated from one another, or things which had been separated are joined, or things which were here pass over there, or things which are now come to subsist then; in all which the being of things suffers no loss. Of these it has been said: “All things that arise decay, and things that grow, grow old”; because all the works of nature, as they have a beginning, so are also not strangers to an end. Of these it has been said:
From nothing nothing, and to nothing nothing can return.
(Persius, Satires III, v.) And because every nature has both a primordial cause and a perpetual subsistence. Of these it has been said:
And it returns into nothing, what was before nothing: because every work of nature, as it flows temporally from a hidden cause into act, so, when by the same act it is destroyed in time, will return to that from which it came.