Here is the key to the teaching of Aristotle that seems to open the doors of its many chambers. In his metaphysics reality is the individual developing from possibility to actuality. In physics individual phenomena get a reality through their development from lower to higher types. In psychology the individual person is real when the particulars, the physiological and psychological states, develop toward the soul, which is their truth. So, too, in the great system of logic in which Aristotle was pioneer, he is simply trying to give the particular judgment a meaning by showing its linkage to the universal judgment. Everywhere the starting-point of Aristotle is the perceptual fact. Everywhere his purpose is to reëstablish it by showing its relation to abiding conception in the individual. It may be well to remark, however, that Aristotle does not altogether succeed in constructing a consistent theory. In spite of his criticism of Plato’s transcendent Ideas, in many places Aristotle does not overcome Plato’s dualism. Frequently he differs from Plato more in words than in meaning. We shall observe some of his inconsistencies in their place. We shall see that Aristotle as he meant to be was different from Aristotle as he was. Aristotle as he meant to be――Aristotle as the opponent of Plato’s dualism――develops a philosophy from a single fundamental principle. Aristotle as he was, reverts at many critical points to Plato’s dualism. Aristotle’s principle of development may appear at first blush very much like the modern principle of evolution. As a matter of fact it was very different. In all Greek philosophy after Socrates the study of morals was fundamental. The ideal of Socrates, Democritus, Plato, Aristotle, and the later Schools was a moral ideal. Being moral it was fixed, and it fixed all the changes of life to it as a centre. Nature was to the Greek a museum of types oscillating around a perfect form. There was no evolution in the sense of progress. There was development within the individual――the boy becomes a man, the seed becomes a flower; but there was no evolution from genus to genus. Indeed, any variation of the individual from its type was considered a defect.