THE THIRD PERIOD begins with the return of the philosopher to his native city. It unites the completeness of form belonging to the first with the profounder characteristical content belonging to the second. The memories of his youthful years seem at this time to have risen anew before the soul of Plato, and to have imparted again to his literary activity the long lost freshness and fulness of that period, while at the same time his abode in foreign lands, and especially his acquaintance with the Pythagorean philosophy, had greatly enriched his mind with a store of images and ideals. This reviving of old memories is seen in the fact that the writings of this group return with fondness to the personality of Socrates, and represent in a certain degree the whole philosophy of Plato as the exaltation of the doctrine and the ideal embodiment of the historical character of his early master. In opposition to both of the first two periods, the third is marked externally by an excess of the mythical form connected with the growing influence of Pythagoreanism in this period, and internally by the application of the doctrine of ideas to the concrete spheres of psychology, ethics and natural science. That ideas possess objective reality, and are the foundation of all essentiality and truth, while the phenomena of the sensible world are only copies of these, was a theory whose vindication was no longer attempted, but which was presupposed as already proved, and as forming a dialectical basis for the pursuit of the different branches of science. With this was connected a tendency to unite the hitherto separate branches of science into a systematic whole, as well as to mould together the previous philosophical directions, and show the inner application of the Socratic philosophy for ethics, of the Eleatic for dialectics, and the Pythagorean for physics. Upon this standpoint, the Phædrus, Plato’s inaugural to his labors in the Academy, together with the Symposium, which is closely connected with it, attempts to subject the rhetorical theory and practice of their time to a thorough criticism, in order to show in opposition to this theory and practice, that the fixedness and stability of a true scientific principle could only be attained by grounding every thing on the idea. On the same standpoint the Phædon attempts to prove the immortality of the soul from the doctrine of ideas; the Philebus to bring out the conception of pleasure and of the highest good; the Republic to develop the essence of the state, and the Timæus that of nature. Having thus sketched the inner development of the Platonic philosophy, we now turn to a systematic statement of its principles.