Demetrius

Those who first assumed that the arts are like the bodily senses, seem to me to have perceived very clearly the power of making distinctions which both possess, by which power we are enabled to apprehend opposites, as well in the one case as in the other. For the arts and the senses have this power in common; though in the use to which we put the distinctions made, they differ.

For our sense-perception has no greater facility in distinguishing white objects than black, or sweet things than bitter, or soft and yielding substances than hard and resisting ones, but its function is to receive impressions from all objects alike, and having received them, to report the resulting sensation to the understanding. The arts, on the other hand, which proceed by the use of reason to the selection and adoption of what is appropriate, and to the avoidance and rejection of what is alien to themselves, contemplate the one class of objects with direct intent and by preference, and, yet incidentally contemplate the other class also, and in order to avoid them.

For instance, the art of healing has incidentally studied the nature of disease, and the art of harmony the nature of discord, in order to produce their opposites; and the most consummate arts of all, namely, temperance, justice, and wisdom, since their function is to distinguish, not only what is good and just and expedient, but also what is bad and unjust and disgraceful, have no praises for a guilelessness which plumes itself on its inexperience of evil, nay, they consider it to be foolishness, and ignorance of what ought especially to be known by men who would live aright.