[1] Time is that wherein there is opportunity, and opportunity is that wherein there is no great time. Healing is a matter of time, but it is sometimes also a matter of opportunity. However, knowing this, one must attend in medical practice not primarily to plausible theories, ¹ but to experience combined with reason. For a theory is a composite memory of things apprehended with sense-perception. For the sense-perception, coming first in experience and conveying to the intellect the things subjected to it, is clearly imaged, and the intellect, receiving these things many times, noting the occasion, the time and the manner, stores them up in itself and remembers. Now I approve of theorising also if it lays its foundation in incident, and deduces its conclusions in accordance with phenomena. For if theorising lays its foundation in clear fact, it is found to exist in the domain of intellect, which itself receives from other sources each of its impressions. So we must conceive of our nature as being stirred and instructed under compulsion by the great variety of things; and the intellect, as I have said, taking over from nature the impressions, leads us afterwards into truth. But if it begins, not from a clear impression, but from a plausible fiction, ² it often induces a grievous and troublesome condition. All who so act are lost in a blind alley. Now no harm would be done if bad practitioners received their due wages. But as it is their innocent patients suffer, for whom the violence of their disorder did not appear sufficient without the addition of their physician’s inexperience. I must now pass on to another subject.