[187] 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.