The Six Books of Proclus on the Theology of Plato, Volume 2

For if all progressions are through similitude, and things more similar to first natures subsist in union with them prior to such as are dissimilar, but it is impossible for things which are generated in a part of time to be conjoined with eternal natures, (for as being generated they differ from first natures which are self-subsistent,[235] and as existing once, they are separated from things which always exist, but the media between these, are such things as are partly similar and partly dissimilar to them)—this being the case, the medium between things which are once generated, and those that exist always, is either that which is always becoming to be, or that which is once, or that which is not truly being. It is however impossible it should be that which once only truly is. And that which is once not truly being is the same with that which is becoming to be. Hence the medium is not that which is once only. It remains, therefore, that the medium between both is that which is always becoming to be, being conjoined indeed with the worse of the two through becoming to be, but through subsisting always, imitating an eternal nature.

From these things it is evident that perpetuity is twofold, the one indeed being eternal, but the other subsisting according to time. The one also being a stable, but the other a flowing perpetuity. And the one indeed having its being collected, and the whole subsisting at once, but the other diffused, and expanded according to temporal extension. And the one being a whole of itself, but the other consisting of parts, each of which is separate, according to prior and posterior.