The elaboration of another class of abstract ideas, those of quality, marched at first by a parallel road. To a limited extent such a parallel march is possible. The words which are used to express sensible qualities suggest the same ideas to different minds. They are applied by different minds to the same objects. But the limits of such an agreement are narrow. When we pass from the abstract ideas of qualities, or generalizations as to substances, which can be tested by the senses, to such ideas as those, for example, of courage or justice, law or duty, though the words suggest, on the whole, the same ideas to one man as to another, not all men would uniformly apply the same words to the same actions. The phenomena which suggest such ideas assume a different form and colour as they are regarded from different points of view. They enter into different combinations. They are not sharply marked out by lines which would be universally recognized. The attention of different men is arrested by different features. There is consequently no universally recognized definition of them. Nor is such a definition possible. The ideas themselves tend to shade off into their contraries. There is a fringe of haze round each of them. The result is that assertions about them vary. There is not one system of philosophy only; there are many. * * * * * Between these two classes of generalizations and abstractions, those of quantity and those of quality or substance, many Greek thinkers do not appear to have made any clear distinction. Ideas of each class were regarded as equally capable of being defined; the canons of inference which were applicable to the one were conceived to be equally applicable to the other: and the certainty of inference and exactness of demonstration which were possible in regard to the ideal forms of geometry, were supposed to be also possible in regard to the conceptions of metaphysics and ethics.[204] The habit of making definitions, and of drawing deductions from them, was fostered by the habit of discussion. Discussion under the name of dialectic, which implies that it was but a regulated conversation, had a large place, not only in the rhetorical and philosophical schools, but also in ordinary Greek life. It was like a game of cards. The game, so to speak, was conducted under strict and recognized rules; but it could not proceed unless each card had a determined and admitted value. The definition of terms was its necessary preliminary; and dialectic helped to spread the habit of requiring definitions over a wider area and to give it a deeper root.