The soul through all her being is immortal, for that which is ever in motion is immortal; but that which moves another and is moved by another, in ceasing to move ceases also to live. Only the self-moving, never leaving self, never ceases to move, and is the fountain and beginning of motion to all that moves besides. Now, the beginning is unbegotten, for that which is begotten has a beginning; but the beginning is begotten of nothing, for if it were begotten of something, then the begotten would not come from a beginning. But if unbegotten, it must also be indestructible; for if beginning were destroyed, there could be no beginning out of anything, nor anything out of a beginning; and all things must have a beginning. And therefore the self-moving is the beginning of motion; and this can neither be destroyed nor begotten, else the whole heavens and all creation would collapse and stand still, and never again have motion or birth. But if the self-moving is proved to be immortal, he who affirms that self-motion is the very idea and essence of the soul will not be put to confusion. For the body which is moved from without is soulless; but that which is moved from within has a soul, for such is the nature of the soul. But if this be true, must not the soul be the self-moving, and therefore of necessity unbegotten and immortal? Enough of the soul's immortality.
—Plato, Phaedrus, p. -1