This, as Professor Jowett remarks, "remains a difficulty for us as well as for the Greeks of the fourth century before Christ, and is the stumbling-block of Kant's _Critic_, and of the Hamiltonian adaptation of Kant as well as of the Platonic ideas. It has been said that 'you cannot criticise Revelation.' 'Then how do you know what is Revelation, or that there is one at all?' is the immediate rejoinder. 'You know nothing of things in themselves.'--'Then how do you know that there are things in themselves?' In some respects the difficulty pressed harder upon the Greek than upon ourselves. For conceiving of God more under the attribute of knowledge than we do, he was more under the necessity of {159} separating the divine from the human, as two spheres which had no communication with one another." Next follows an extraordinary analysis of the ideas of 'Being' and 'Unity,' remarkable not only for its subtlety, but for the relation which it historically bears to the modern philosophic system of Hegel. "Every affirmation is _ipso facto_ a negation;" "the negation of a negation is an affirmation;" these are the psychological (if not metaphysical) facts, on which the analysis of Parmenides and the philosophy of Hegel are both founded. We may pass more rapidly by the succeeding dialogues of the series: the _Theaetetus_ (already quoted from above, p. 89), which is a close and powerful investigation of the nature of knowledge on familiar Platonic lines; the _Sophist_, which is an analysis of fallacious reasoning; and the _Statesman_, which, under the guise of a dialectical search for the true ruler of men, represents once more Plato's ideal of government, and contrasts this with the ignorance and charlatanism of actual politics. In relation to subsequent psychology, and more particularly to the logical system of Aristotle, these dialogues are extremely important. We may indeed say that the systematic logic of Aristotle, as contained in the _Organon_, is little more than an abstract {160} or digest of the logical theses of these dialogues. Definition and division, the nature and principle of classification, the theory of predication, the processes of induction and deduction, the classification and criticism of fallacies,--all these are to be found in them. The only addition really made by Aristotle was the systematic theory of the syllogism.