Marsilio Ficino’s Platonic Theology, Volume II

Every rational soul is immortal. First argument: because it moves itself and moves in a circle.

1We place all souls in the order of the third essence and maintain by this general argument that they are immortal: their motion is primary, and because it is primary, it takes a circular course. Since, then, their motion is primary, it is also perpetual, for wherever motion is primary, it is perpetual. If the fountain of motion were to run dry, nothing further would move in nature. Again, whatever has primary motion supplies itself with both inward and outward motion. Inward motion is life; thus it supplies itself with life. And because it never abandons itself—since every nature has within it a perpetual love of itself—it never ceases to live. For if what is moved by another remains in motion so long as it cleaves to its mover, then still more must whatever moves itself remain in motion. Since it is identical with its mover and is never deserted by that mover, it never ceases to move. Moreover, suppose it were to perish at some time. Either it would cease to exist before it ceased to move, cease to move before it ceased to exist, or cease both to exist and to move at the same instant. The first is impossible, because motion cannot remain apart from essence; so is the second, because self-motion is the constant companion of whatever originates its own motion; nor can the third occur.

2For no destructive change can ever break out within it from that inward principle which gives rise to its vital and life-giving motion. Nor can such a change come from without, for the fountain of motion does not receive its motion from elsewhere. This, then, is a property of the third essence. And since, under this common definition, all rational souls belong to that essence, the same property belongs to them all.

3They are immortal for this further reason: by the nature of the third essence and of primary motion, they are carried in a circle.