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 with what is unseemly, what is just and generally good, by placing it in juxta-position with what is unjust and bad. And, indeed, if any one considers everything that there is in the world, he will be able to arrive at a proper estimate of its character, by taking it in the same manner; for each separate thing is by itself incomprehensible, but by a comparison with another thing, is easy to understand it.

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.