THE SECOND PERIOD—the dialectic or the Megaric—is marked externally, by a less prominence of form and poetic contemplation, and not unfrequently indeed, by obscurity and difficulties of style, and internally, by the attempt to give a satisfactory mediation for the Eleatic doctrine and a dialectic foundation for the doctrine of ideas. By his exile at Megara, and his journeys to Italy, Plato became acquainted with other and opposing philosophical directions, from which he must now separate himself in order to elevate the Socratic doctrine to its true significance. It was now that he first learned to know the philosophic theories of the earlier sages, for whose study the necessary means could not at that period, so wanting in literary publicity, be found at Athens. By his separation from these varying standpoints, as his older fellow pupils had already striven to do, he attempted striding over the narrow limits of ethical philosophizing, to reach the final ground of knowing, and to carry out the art of forming conceptions as brought forward by Socrates, to a science of conceptions, _i. e._ to the doctrine of ideas. That all human acting depends upon knowing, and that all thinking depends upon the conception, were results to which Plato might already have attained through the scientific generalization of the Socratic doctrine itself, but now to bring this Socratic wisdom within the circle of speculative thinking, to establish dialectically that the conception in its simple unity is that which abides in the change of phenomena, to disclose the fundamental principles of knowledge which had been evaded by Socrates, to grasp the scientific theories of the opposers direct in their scientific grounds, and follow them out in all their ramifications,—this is the problem which the Megaric family of dialogues attempts to solve. The Theatætus stands at the head of this group. This is chiefly directed against the Protagorean theory of knowledge, against the identification of the thinking and the sensible perception, or against the claim of an objective relativity of all knowledge. As the Gorgias before it had sought to establish the independent being of the ethical, so does the Theatætus ascending from the ethical to the theoretical, endeavor to prove an independent being and objective reality for the logical conceptions which lie at the ground of all representation and thinking, in a word, to prove the objectivity of truth, the fact that there lies a province of thought immanent in the thinking and independent of the perceptions of the senses. These conceptions, whose objective reality is thus affirmed, are those of a species, likeness and unlikeness, sameness and difference, &c.