If the second is the great constructive period of Plato's life, the third may be described as his systematic and synthetic period. Every part of his philosophy is here linked up with every other part. All the details of the system are seen to flow from the one central principle of his thought, the theory of Ideas. Every sphere of knowledge and being is in turn exhibited in the light of that principle, is permeated and penetrated by it. The plan for expounding Plato which first suggests itself is to go through the dialogues, one by one, and extract the doctrine of each successively. But this suggestion has to be given up as soon as it is mentioned. For although the philosophy of Plato is in itself a systematic and coherent body of thought, he did not express it in a systematic way. On the contrary, he scatters his ideas in all directions. He throws them out at random in any order. What logically comes first often appears last. It may be found at the end of a dialogue, and the next step in reasoning may make its appearance at the beginning, or even in a totally different dialogue. If, therefore, we are to get any connected view of the system, we must abandon Plato's own order of exposition, and piece the thought together for ourselves. We must begin {177} with what logically comes first, wherever we may find it, and proceed with the exposition in the same manner. A similar difficulty attends the question of the division of Plato's philosophy. He himself has given us no single and certain principle of division. But the principle usually adopted divides his philosophy into Dialectic, Physics, and Ethics. Dialectic, or the theory of Ideas, is Plato's doctrine of the nature of the absolute reality. Physics is the theory of phenomenal existence in space and time, and includes therefore the doctrine of the soul and its migrations, since these are happenings in time. Ethics includes politics, the theory of the duty of man as a citizen, as well as the ethics of the individual. Certain portions of the system, the doctrine of Eros, for example, do not fall very naturally into any of these divisions. But, on the other hand, though some dialogues are mixed as to their subject matter, others, and those the most important, fall almost exclusively into one or other division. For example, the "Timaeus," the "Phaedo," and the "Phaedrus," are physical. The "Philebus," the "Gorgias," and the "Republic," are ethical. The "Theaetetus," the "Sophist," and the "Parmenides," are dialectical. 2. The Theory of Knowledge.