We have seen that Plato sets the world of ideas apprehended by reason over against the world of phenomena, known to us through our senses. The latter world is material, the former immaterial. This concept of the immateriality of ideas was something new in philosophy. Anaxagoras had thought his formative principle (Νοῦς), as his predecessors had thought theirs, to be as material as the “seeds” out of which all things were made; but Plato developed an immaterial, an ideal world, wherein are found all cause and all reality. Now the Platonic ideas are apprehended by the human intellect. What are the consequences of this fact? It must follow that man’s reason has a nature similar to that of the ideas; like them it must belong to the world which is above the senses; and with them it must partake of the Absolute. But Plato shows that the ideas are eternal and immortal, and draws therefrom the logical conclusion that man’s intellect, his reasoning soul, likewise knows no creation and is free from death. However he is not content to let the matter rest on this argument alone, but he supports the doctrine of immortality by many proofs, as in the Phaedrus where Socrates explains: “The soul through all her being is immortal, for that which is ever in motion is immortal; but that which moves another and is moved by another, in ceasing to move, ceases also to live. Only the self-moving, as it never leaves itself, never ceases to move and is the fountain and beginning of motion to all that moves besides.”[191] In the Phaedo[192] he represents Socrates as offering a number of different arguments to his questioning friends. One of these is that by which he first proves that souls exist before they are domiciled in our bodies, for recollection implies a previous existence, and since men can recall and recognize things which they have never seen or been taught in their present existence, it follows that they have been born with this knowledge, so that what we call learning is after all only a recollection of ideas gained in a previous existence. Socrates concludes his argument with the question: “Then may we not say, Simmias, that if, as we are always repeating, there is an absolute beauty, and goodness, and an absolute essence of all things; and if to this, which is now discovered to have existed in our former state, we refer all our sensations, and with this compare them, finding these ideas to be pre-existent and our inborn possession—then our souls must have had a prior existence, but if not, there would be no force in the argument?” To this his hearers give ready assent. In the Meno this same argument is very adroitly drawn from the realm of mathematics.[193] An untutored slave is made to “recollect” that the square of the hypothenuse of an isosceles right-angled triangle is equal to twice the square of one of its sides. This is the doctrine of recollection to which Wordsworth has given beautiful expression in his familiar lines: