Plato therefore advanced the doctrine of reminiscence, or as he called it, _Anamnesis_, as proof of our pre-existence. Knowledge is recollection. The Ideas have always been present in the mind, and when we recognize them we have knowledge. The Ideas have no past or future, but they always exist. It is the mind that undergoes awakening――an awakening to their existence in itself. When the mind sees the objects of physical nature, it awakens in painful astonishment at the contrast between the sense world and the Ideas of its native world of immateriality. In a mythical representation in the _Phædrus_, Plato supposes that before the present life our souls have beheld the pure Ideas in their full reality, that the Ideas had been forgotten in our birth into the present life, but that the perception of similar corporeal things calls the soul back to the Ideas themselves. Then the “Eros” is awakened――the native philosophical impulse or inborn love for the Ideas, by which the soul is raised again to the knowledge of that true reality. Only the pure Ideas themselves will satisfy this longing; the embodiment of the Ideas in art or personalities is not adequate. The Eros ties us to the Ideas. God does not have this longing, for He fully knows the Good. The ignorant man does not have this longing, for he does not suspect the existence of the Ideas in himself. The Eros is the homesickness that the lover of the truth feels. Our birth is but a sleep and a forgetting, The soul that rises in us, our life’s star, Hath had elsewhere its setting, And cometh from afar; Not in entire forgetfulness, And not in utter nakedness, But trailing clouds of glory do we come From God, who is our home.[31] When, in the _Meno_, the Sophistic dilemma was proposed to Socrates, “How can inquiry be made into what we know or into what we don’t know?” Socrates pointed out that the only escape from the dilemma was the process of recollecting, and that knowledge is the thing recalled. Socrates then called a slave to him, and by skillfully questioning him found that the slave recognized the mathematical relationship between the square on the hypothenuse of a right triangle and the sums of the squares on the other two sides. “The ignorant slave can only have been recollecting,” says Socrates. Mathematical knowledge is extracted from the sense-perception of the slave only because the slave has through such perception the opportunity of recollecting Ideas present in himself and not hitherto suspected by himself. In Plato’s system, mathematical forms have an important place. They are the links by means of which the Idea shapes space teleologically into the sense world.