The theory of Ideas is itself based upon the theory of knowledge. What is knowledge? What is truth? Plato opens the discussion by telling us first what knowledge and truth are not. His object here is the refutation of false theories. These must be disposed of to clear the ground preparatory to positive exposition. The first such false theory which he attacks is that knowledge {178} is perception. To refute this is the main object of the "Theaetetus." His arguments may be summarized as follows:-- (1) That knowledge is perception is the theory of Protagoras and the Sophists, and we have seen to what results it leads. What it amounts to is that what appears to each individual true is true for that individual. But this is at any rate false in its application to our judgment of future events. The frequent mistakes which men make about the future show this. It may appear to me that I shall be Chief Justice next year. But instead of that, I find myself, perhaps, in prison. In general, what appears to each individual to be the truth about the future frequently does not turn out so in the event. (2) Perception yields contradictory impressions. The same object appears large when near, small when removed to a distance. Compared with some things it is light, with others heavy. In one light it is white, in another green, and in the dark it has no colour at all. Looked at from one angle this piece of paper seems square, from another it appears to be a rhombus. Which of all these impressions is true? To know which is true, we must be able to exercise a choice among these varying impressions, to prefer one to another, to discriminate, to accept this and reject that. But if knowledge is perception, then we have no right to give one perception preference over another. For all perceptions are knowledge. All are true. (3) This doctrine renders all teaching, all discussion, proof, or disproof, impossible. Since all perceptions are equally true, the child's perceptions must be just as much the truth as those of his teachers. His teachers, {179} therefore, can teach him nothing. As to discussion and proof, the very fact that two people dispute about anything implies that they believe in the existence of an objective truth. Their impressions, if they contradict each other, cannot both be true. For if so, there is nothing to dispute about. Thus all proof and refutation are rendered futile by the theory of Protagoras. (4) If perception is truth, man is the measure of all things, in his character as a percipient being. But since animals are also percipient beings, the lowest brute must be, equally with man, the measure of all things. (5) The theory of Protagoras contradicts itself. For Protagoras admits that what appears to me true is true. If, therefore, it appears to me true that the doctrine of Protagoras is false, Protagoras himself must admit that it is false.