Plato――Objective Idealism. Being consists of permanent moral and æsthetic concepts or types. In mediæval philosophy Platonism was called realism and was summed up by the phrase _Universalia ante rem_. Aristotle――Conceptualism. The abiding Being does not consist of material atoms nor in spiritual types apart from matter, but is an unfolding essence _in_ matter. This was usually called conceptualism by the Schoolmen, and was summed up by the phrase _Universalia in re_. Aristotle’s conception was as difficult as it was important. He was not always clearly a conceptualist, but sometimes appeared in the rôle of an “objective realist.” =Democritus and Plato――Their Similarities and Differences.= The materialism of Democritus and the idealism of Plato were as opposed as was possible within the realm of Greek thought. We must not exaggerate their similarities, but they had at least four common characteristics. _Their Similarities._ 1. Both develop an outspoken rationalism,[22] which starts as a reaction from the perception theory of Protagoras. They agree with Protagoras that perception cannot yield truth, and so they turn away from perception to the reason to find true knowledge. 2. Both develop a world of twofold reality. Perceptions are not regarded by them as illusions, although perceptions are transitory. Both make a new estimate of perceptions, and give to the world of perceptions a relative value. There are therefore two kinds of reality: the relative reality of the world of perceptions and the absolute reality of the world of reason. The result in both is a broad theory of knowledge. 3. In both, reality consists in a plural number of objective norms. Both reach their conception of these norms in the same way. The changing qualities of things are stripped away and the true reality is discovered beneath. Both designate this true form by the same word, idea (ἰδέα). To both, the forms are objective entities. 4. Both are attempts to overcome scientifically the dualism which had emerged from the former hylozoism of Greek thought.