Phaedrus

Socrates. For every body which derives motion from without is soulless, but that which has its motion within itself has a soul, since that is the nature of the soul; but if this is true,— that that which moves itself is nothing else than the soul,—then the soul would necessarily be ungenerated and immortal. Concerning the immortality of the soul this is enough; but about its form we must speak in the following manner. To tell what it really is would be a matter for utterly superhuman and long discourse, but it is within human power to describe it briefly in a figure; let us therefore speak in that way. We will liken the soul to the composite nature of a pair of winged horses and a charioteer. Now the horses and charioteers of the gods are all good and of good descent, but those of other races are mixed; and first the charioteer of the human soul drives a pair, and secondly one of the horses is noble and of noble breed, but the other quite the opposite in breed and character. Therefore in our case the driving is necessarily difficult and troublesome. Now we must try to tell why a living being is called mortal or immortal. Soul, considered collectively, has the care of all that which is soulless, and it traverses the whole heaven, appearing sometimes in one form and sometimes in another; now when it is perfect and fully winged, it mounts upward and governs the whole world; but the soul which has lost its wings is borne along until it gets hold of something solid, when it settles down, taking upon itself an earthly body, which seems to be self-moving, because of the power of the soul within it; and the whole, compounded of soul and body, is called a living being, and is further designated as mortal. It is not immortal by any reasonable supposition, but we, though we have never seen or rightly conceived a god, imagine an immortal being which has both a soul and a body which are united for all time. Let that, however, and our words concerning it, be as is pleasing to God; we will now consider the reason why the soul loses its wings. It is something like this. The natural function of the wing is to soar upwards and carry that which is heavy up to the place where dwells the race of the gods. More than any other thing that pertains to the body it partakes of the nature of the divine. But the divine is beauty, wisdom, goodness, and all such qualities; by these then the wings of the soul are nourished and grow, but by the opposite qualities, such as vileness and evil, they are wasted away and destroyed.