Demetrius (1.1-1.2)

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.