De Ebrietate

For quantity in such compounds is measured by fixed limits and rules, and it is not safe either to stop short before one has reached them, nor to advance beyond them. For if too little be applied, it relaxes, and if too much, it strains the natural powers; and each extremity is mischievous, the one from its impotence being capable of producing any effect at all, and the other by reason of its exceeding strength being necessarily hurtful. Again it is very plain with reference to smoothness, and roughness, and thickness, and close compression, or on the other hand leanness and slackness, how very much influence all these differences have in respect of doing good or harm.

Nor indeed is any one ignorant that scarcely anything whatever of existing things, if you consider it in itself and by itself, is accurately understood; but by comparing it with its opposite, then we arrive at a knowledge of its true nature. As for instance, we comprehend what is meant by little by placing it in juxta-position with what is great; we understand what dry is by comparing it with wet, cold by comparing it with heat, light by comparing it with heavy, black by contrasting it with white, weak by contrasting it with strong, and few by comparing it with many. In the same way also, in whatever is referred to virtue or to vice,

what is advantageous is recognised by a comparison with what is injurious, what is beautiful by a comparison

Now, that which is unable to bear witness to itself, but which stands in need of the advocacy of something else, is not to be trusted or thought steady. So that in this way those men are convicted who say that they have no difficulty in assenting to or denying propositions about anything.