“How is it, that whereas the body is mortal by nature, man reasons on the things of immortality, and often, where virtue demands it, courts death? Or how, since the body lasts only for a time, does man imagine eternal things, so as to despise what lies before him, and desire what is beyond ? The body could not spontaneously have such thoughts about itself, nor could it consider what is external to itself. For it is mortal and lasts only for a time. And it follows that that which thinks what is opposed to the body and against its nature must be distinct in kind. What then can this be, save a rational and immortal soul? For it introduces the echo of higher things, not outside, but within the body, as the musician does with his lyre. Or how again, do the eye and the ear being naturally constituted to see and to hear, turn from some objects and choose others ? For who is it that turns away the eye from seeing? Or who shuts off the ear from hearing, its natural function ? Or who often hinders the palate, for which it is natural to taste things, from its natural impulse ;
Or who withholds the hand from its natural activity of touching something, or turns aside the sense of smell from its normal exercise ? Who is it that thus acts against the natural instincts of the body ? Or how does the body, turned from its natural course, turn to the counsels of another and allow itself to be guided at the beck of that other? Why, these things simply prove that the rational soul presides over the body. For the body is not even constituted to drive itself, but it is driven by another’s will, just as a horse does not harness himself, but is driven by his master. Hence laws for human beings to practise what is good and to abstain from evil-doing, while for animals evil remains unthought of and undiscerned, because they lie outside rationality and the process of understanding. I think then that the existence of a rational soul in man is proved by what we have said.
A further point in the Church’s teaching which you must know, to show how the idols are to be overthrown, is the immortality of the soul. But we shall more directly reach this knowledge from what we know of the body, and from the difference between the body and the soul. For if our argument has proved it to be distinct from the body, while the body is by nature mortal, it follows that the soul is immortal, because it is not like the body. And again, if as we have shown, the soul moves the body and is not moved by other things, it follows that the movement of the soul is spontaneous, and that this spontaneous movement goes on after the body is dead. If then the soul were moved by the body, it would follow that cutting its motor would involve its death. But if the soul also moves the body, it follows all the more that it moves itself. But if moved by itself, it follows that it outlives the body. For the movement of the soul is the same thing as its life, just as, of course, we call the body alive when it moves, and say that its death takes place when it stops moving. But this can be made clearer once for all from the action of the soul in the body. For if even when united and coupled with the body it is not shut in or commensurate with the small dimensions of the body, but often, when the body lies in bed, not moving, but in death-like sleep, the soul keeps awake by virtue of its own power, and transcends the natural power of the body, and as though travelling away from the body while remaining in it, imagines and contemplates spiritual realities and often even converses with the saints and angels who are beyond earthly and bodily existence, and approaches them in the confidence of the purity of its intelligence ; shall it not all the more, when separated from the body at the time appointed by God Who joined them, have a clearer knowledge of immortality? For if even when joined to the body it lived a life outside the body, much more shall its life continue after the death of the body, and live without ceasing by reason of God Who made it thus by His own Word, our Lord Jesus Christ. . And just as, the body being mortal, its senses also have mortal things as their objects, so, since the soul contemplates and beholds immortal things, it follows that it is immortal and lives for ever. For ideas and thoughts about immortality never desert the soul, but abide in it, and are as it were the fuel in it which ensures its immortality. This then is why the soul has the capacity for beholding God, and is its own way thereto, receiving not from without but from herself the knowledge and apprehension of the Word of God.”
St. Athanasius, Against the Heathen, 32 – 33.