The dialogue then passes to a discussion of good and bad writing and speaking. The truth is the sole criterion of value, and this can be obtained only by definition; next there must be orderly arrangement, a beginning, a middle and an end. In rhetoric it is absolutely essential for a man to study human nature first; he cannot hope to persuade an audience if he is unaware of the laws of its psychology; not all speeches suit all audiences. Further, writing is inferior to speaking, for the written word is lifeless, the spoken is living and its author can be interrogated. It follows then that orators are of all men the most important because of the power they wield; they will be potent for destruction unless they love the truth and understand human nature; in short, they must be philosophers. The like of this had nowhere been said before. It opened a new world to human speculation. First, the teaching about oratory is of the highest value. Plato's quarrel with the sophists was based on their total ignorance of the enormous power they exercised for evil, because they knew not what they were doing. They professed to teach men how to speak well, but had no conception of the science upon which the art of oratory rests. In short, they were sheer impostors. Even Aristotle had nothing to add to this doctrine in his treatise on _Rhetoric_, which contains a study of the effects which certain oratorical devices could be prophesied to produce, and provides the requisite scientific foundation. Again, the indifference to or the ridicule of truth shown by some sophists made them odious to Plato. He would have none of their doctrines of relativity or flux. Nothing short of the Absolute would satisfy his soaring spirit. He was sick of the change in phenomena, the tangible and material objects of sense. He found permanence in a world of eternal ideas. These ideas are the essence of Platonism. They are his term for universal concepts, classes; there are single tangible trees innumerable, but one Ideal Tree only in the Ideal world beyond the heavens. Nothing can possibly satisfy the soul but these unchanging and permanent concepts; it is among them that it finds its true home. Lastly, the tripartite division of the nature of the soul here first indicated is a permanent contribution to philosophy. Thus Plato's system is definitely launched in the _Phaedrus_. His subsequent dialogues show how he fitted out the hulk to sail on his voyages of discovery.