[8] For as the old story tells us, God, when he had combined pleasure and pain, two things naturally at variance, under one head, gave to us an outward sense capable of appreciating them both, not at the same moment, but at different times, fixing the period of the return of one to be simultaneous with the moment of the flight of the other. Thus from one root of the dominant principle, the two shoots of virtue and vice sprang up, neither blossoming nor bearing fruit at the same time;