Marsilio Ficino’s Platonic Theology, Volume II

15Nor does a body increase or diminish itself. For neither can occur without alteration and without a kind of generation and corruption. We grow and diminish through changes of qualities: while we grow, new bodily parts are generated in us; while we diminish, old ones are corrupted. If, then, increase and decrease require alteration, generation, and corruption, a body that does not move itself through alteration, generation, or corruption does not move itself by increasing or diminishing either. Nor does it condense or rarefy itself, since these motions too do not occur without alteration.

16Someone may perhaps say that a body at least changes its place of itself, either by passing from one place to another or by circular motion. But this too is impossible to maintain. For if body is powerless with respect to the preceding motions, which are internal, it will be still more powerless with respect to external motions. By external motions I mean circular motion and passage, which extend to termini more remote than those of internal motions. Thus, if a body cannot of itself be moved to its own termini, how could it be moved to termini that belong to something else? Let us therefore now conclude as follows. No motions appear in bodies other than the eight already enumerated. Through none of them are bodies driven by themselves. Therefore bodies are in no way moved from themselves.

17Yet bodies are moved, as everyone knows, by a certain nature acting from within, even without external impulses. No Platonist will deny that this nature is the soul, which can move itself because motions of another nature, beyond those eight, are found in it. Insofar as the part by which the body is governed is concerned, the soul contains a temporal succession of seeds arising within it now in one fashion and now in another to generate corporeal things. Insofar as that part of the soul which is occupied in cognition is concerned, it contains a temporal passage from one cognition and affection to another: a temporal ascent from the lowest effects of nature through intermediate causes to the highest principles, and conversely a descent from the highest principles through intermediate causes to the lowest effects.