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

For if every intellectual form is eternal and immoveable, it is essentially the cause of immutable and eternal hypostases, but not of such as are generated and corrupted. So that every thing which subsists according to an intellectual form is an eternal intellectual nature. For if all forms produce things posterior to themselves by their very being, but their essence possesses an invariable sameness of subsistence, the things produced by them will also be invariably the same, and will be eternal. Hence, neither the genera which subsist from a formal cause, according to a certain time, nor corruptible natures so far as they are corruptible, have a pre-existent intellectual form. For they would be incorruptible and unbegotten, if they derived their hypostasis from intellectual forms.