The perception theory of Protagoras was the starting-point of both Democritus and Plato. Both adopted it in order to transcend it and make it of real significance. Democritus, upon the basis of his materialistic psychology, admitted that sense-perception is only a transitory process, and its knowledge must be as transitory. But he did not agree with Protagoras that all knowledge is perceptual. Sense-perception does yield only relative knowledge; but there is another kind of knowledge that is not relative but absolute. This is knowledge of the reason. Human beings have reason as well as sense-perception. Thus is Democritus a rationalist, although a materialist. The contribution of Democritus to the theory of knowledge consists in just this turn which he gave to Protagoras’ doctrine of perception. The relativity of perception becomes in the Democritan theory a different thing from what it was in the doctrine of the great Sophist. To Protagoras perceptual knowledge is relative, and therefore of no value in determining what is real. To Democritus perceptual knowledge is relative, but it has a value,――a relative value. It gets this relative value from the fact that the reason can determine absolute reality. Perception is the contributor to the reason, and also in turn is illuminated by the reason. In the same breath we may say that Protagoras was a contributor to the theory of Democritus, and in turn that the Protagorean relativism was illuminated by the Democritan rationalism. The result was a twofold knowledge――in the language of Democritus, “genuine knowledge” and “obscure insight.” The objects corresponding to these two kinds of knowledge must be of two kinds. On the one hand, the objects of the reason, or “genuine knowledge,” are the genuine, primary, or real properties of the atoms――for the atoms are reality to Democritus. These are form, size, inertia, density, and hardness.[25] A study of these properties of things is, therefore, a study of real objects. On the other hand, the objects of perception or “obscure insight” are the properties of atoms as perceived obscurely by the senses. These are color, sound, taste, smell. They are the qualities or relative properties of things. A study of these is a study of only what is relatively real. When materialism was revived by the Renaissance, the former group of objects were called “primary qualities” and the latter “secondary qualities.” These terms have become classic, and have rendered permanent Democritus’ evaluation of the objects of the two kinds of knowledge. Out of the fragments of the teaching of the Cosmologists and the one-sided epistemology of the Sophists, Democritus constructed contemporaneously with Plato, perhaps antecedently to him, a theory of twofold knowledge.