=Plato’s Doctrine of Immortality.= Beginning with this conception of the dual nature of the human soul, Plato reasons both backward and forward from it: backward to its pre-existence, and forward from its post-existence,――its existence after death. In the _Phædo_, Plato has put into the mouth of what has become his Platonized Socrates his final thought concerning the relation of this present life to its past and its future. It is plainly the doctrine of the transmigration of souls, which he got from the Pythagoreans. The soul has a reality that is imperishable, and the soul is rewarded or punished for its conduct in one existence by the kind of existence into which it is metamorphosed. In prison, on that fatal day when he drank the poison, Socrates explained to those around him why he was so cheerful at the thought of death. Is not our present existence a kind of death? Is not the soul in the present life deterred from true knowledge by the trammels of the bodily desires? The true philosopher is he who turns away from his body’s passions,――dies to them, and tries to live the reality of the world of Ideas. We shall have full knowledge when we pass beyond the grave and then we shall be rewarded, if we have striven truly. But at present our body hampers and misleads us with its perceptions of changing mortality around us, and with its transitory desires. This life itself is the reward or punishment for our conduct in our preceding state. =1. The Immortality of Pre-existence.= What proof does Plato offer for our existence before this life? The Ideas, these testimonies of reality, form a part of the human soul. They are eternal, and have not been created by the soul. Knowledge is not the origination of a new truth, but is the recognition of Ideas, whose presence the mind merely records. Greek psychology never got much farther than this. The modern psychological conception of the soul as a dynamic something, which creates its own content, was quite foreign to the Greeks. To Plato, as to all other Greeks, the soul is as passive as the wax that receives the impress of the seal. All Greek psychology was under this general limitation: all ideas must be “given” to the soul. Therefore if the Ideas are not “given” by perception, because perception is of the changing; if nevertheless the soul finds itself in possession of the Ideas on the occasion of perception; if the soul did not create the Ideas, because the soul is by nature passive; the logical and only conclusion is that the soul was already in possession of the Ideas in a pre-existent state. Pre-existence is the only way of accounting for the full-born knowledge of the soul, and it is interesting to note how important was the pre-existent state to the imagination of the ancient world.