Plato says that that which always moves by itself, and has no beginning of motion, also has no end; but that the soul of man always moves by itself, and because it is flexible for reflection, subtle for discovery, easy of perception, adapted to learning, and because it retains the past, comprehends the present, foresees the future, and embraces the knowledge of many subjects and arts, that it is immortal, since it contains nothing which is mixed with the contagion of earthly weight.
Moreover, the eternity of the soul is understood from virtue and pleasure. Pleasure is common to all animals, virtue belongs only to man; the former is vicious, the latter is honourable; the former is in accordance with nature, the latter is opposed to nature, unless the soul is immortal. For in defence of faith and justice, virtue neither fears want, nor is alarmed at exile, nor dreads imprisonment, nor shrinks from pain, nor refuses death; and because these things are contrary to nature, either virtue is foolishness, if it stands in the way of advantages, and is injurious to life; or if it is not foolishness, then the soul is immortal, and despises present goods, because other things are preferable which it attains after the dissolution of the body.
The greatest proof of immortality [is] that man alone has the knowledge of God. In the dumb animals there is no notion of religion, because they are earthly and bent down to the earth. Man is upright, and beholds the heaven for this purpose, that he may seek God. Therefore he cannot be other than immortal, who longs for the immortal. He cannot be liable to dissolution, who is connected with God both in countenance and mind. Finally, man alone makes use of the heavenly element, which is fire. For if light is through fire, and life through light, it is evident that he who has the use of fire is not mortal, since this is closely connected, this is intimately related to Him without whom neither light nor life can exist.
— Lactantius. The Epitome of the Divine Institutes. LXX.