We proceed now to the _Theætetus_, one of the most important, as well as difficult, of the Platonic dialogues. To this Mr. Jowett has written a rather long but excellent Introduction, replete with large views of the Platonic philosophy, and containing many original and striking remarks, _e.g._ (p. 329): 'The Greeks, in the fourth century before Christ, had no words for "subject" and "object," and no distinct conception of them; yet they were always hovering about the question involved in them.' (We should be inclined to say, that the familiar distinction between τὰ νοητὰ and τὰ αἰσθητὰ, to a considerable extent represented our terms 'subjective' and 'objective.') Again (p. 328): 'The writings of Plato belong to an age in which the power of analysis had outrun the means of knowledge; and through a spurious use of dialectic, the distinctions which had been already "won from the void and formless infinite," seemed to be rapidly returning to their original chaos.' And (p. 353), 'The relativity of knowledge' (viz., to the individual mind) 'is a truism to us, but was a great psychological discovery in the fifth century before Christ.' In p. 360 the remark is a shrewd one: 'The ancient philosophers in the age of Plato thought of science' (_i.e._, ἐπιστήμη, exact knowledge) 'only as pure abstraction, and to this _opinion_ (δόξα) stood in no relation.' The subject of _Theætetus_, 'What _is_ knowledge?' involving, as it doubtless does, some satire on Sophists, who professed to teach what they were themselves unable to explain, has been well called 'A critical history of Greek psychology as it existed down to the fourth century.' In this treatise, the views of the earlier philosophers, that there is no test of existence or reality except perception, αἴσθησις, are impugned. Plato did not, perhaps, himself hold the opinion that objective truth existed, independently of opinion; but his favourite theory of ἰδέαι, or abstracts, implied the existence of _some_ typical, eternal, absolute standard of goodness and justice, as well as of the beautiful. If this were not the case, then all moral as well as all physical οὐσίαι would depend on our sense of them. There would be no φύσει δίκαιον, but only νόμῳ δίκαιον. That would be right in every state which the laws enacted; and thus in two neighbouring states one course of acting (say, lying or stealing, or promiscuous intercourse) would be right, because it is legalised; in another it would be wrong, because punishable by the law. Nor is this difficulty wholly imaginary, as Aristotle felt. (Eth. Nic. V. ch. 7.) The old law, for instance, sanctioned polygamy, as modern usage does in some parts of the East; while the law of Europe condemns it. So in the case of murder: a Greek thought it a solemn and absolute duty to slay the slayer of his father; while we should regard it as one murder added to another. There was a good deal of sense therefore in what Protagoras taught, that 'man is the measure,' μέτρον ἄνθρωπος.