The _Philebus_ discusses the question whether Pleasure or Knowledge is the chief good. A metaphysical argument which follows that of the _Parmenides_ ends in the characteristic Greek distinction between the Finite and the Infinite. Pleasure is infinite, because it can exist in greater or less degree; there is a mixed life compounded of finite and infinite and there is a creative faculty to which mind belongs. Pleasure is of two kinds; it is sometimes mixed with pain, sometimes it is pure; the latter type alone is worth cultivating and includes the pleasures of knowledge. Yet pleasure is not an end, but only a means to it. It cannot therefore be the Good, which is an end. Knowledge is at its best when it is dealing with the eternal and immutable, but even then it is not self-sufficient--it exists for the sake of something else, the good. This latter is characterised by symmetry, proportion and truth. Knowledge resembles it far more than even pure pleasure. The _Theaetetus_ discusses more fully the theory of knowledge. It opens with a comparison of the Socratic method to midwifery; it delivers the mind of the thoughts with which it is in travail. The first tentative definition of knowledge is that it is sensation. This is in agreement with the Protagorean doctrine that man is the measure of all things. Yet sensation implies change, whereas we cannot help thinking that objects retain their identity; if knowledge is sensation a pig has as good a claim to be called the measure of all things as a man. Again, Protagoras has no right to teach others if each man's sensations are a law unto him. Nor is the Heracleitean doctrine much better which taught that all things are in a state of flux. If nothing retains the same quality for two consecutive moments it is impossible to have predication, and knowledge must be hopeless. In fact, sensation is not man's function as a reasoning being, but rather comparison. Neither is knowledge true opinion, for this at once demands the demarcation of false opinion or error; the latter is negative, and will be understood only when positive knowledge is determined. Perhaps knowledge is true opinion plus reason; but it is difficult to decide what is gained by adding "with reason", words which may mean either true opinion or knowledge itself, thus involving either tautology or a begging of the question. The dialogue at least has shown what knowledge is not. Locke, Berkeley and Hume, the eighteenth century sensation philosophers, were similarly refuted by Kant. The mind by its mere ability to compare two things proves that it can have two concepts at least before it at the same time, and can retain them for a longer period than a mere passing sensation implies. Yet the problem of knowledge still remains as difficult as Plato knew it to be.