On Drunkenness (186-190)

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.

And why need we wonder? For any one who advances far into matters, and who contemplates them in an unmixed state will know this, that nothing is ever presented to our view according to its real plain nature, but that everything has the most various possible mixtures and combinations.

Some one will say, We at once comprehend colours. How so? Do we not do so by means of the external things, air and light, and also by the moisture which exists in our eyes themselves? And in what way are sweet and bitter comprehended? Is it apart from the moisture in our mouths? And as to all the flavours which are in accordance with, or at variance with nature, are not they in the same case? What, again, are we to say of the smells arising from perfumes which are burnt? Do they exhibit plain unmixed simple natures, or rather qualities compounded of themselves and of the air, and sometimes also of the fire which consumes their bodies, and also of the faculty existing in our own nostrils?