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

For either it possesses both eternally, or both temporally; or the one eternally, but the other temporally. It cannot however, possess both eternally: for it would be an impartible essence, and the nature of soul would in no respect differ from an intellectual hypostasis, viz. a self-motive from an immoveable nature. Nor can it possess both temporally: for thus it would be generated alone, and would neither be self-vital, nor self-subsistent. For nothing which is essentially measured by time is self-subsistent. But soul is self-subsistent. For that which is converted to itself, according to energy, is also essentially converted to itself, and proceeds from itself. It remains therefore, that every soul is partly eternal, and partly participates of time. Either therefore, it is essentially eternal, [but participates of time according to energy,[312]] or vice versa. The latter however, is impossible. Hence, every participable soul, is allotted an eternal essence, but a temporal energy.