As, then, playing at ball not only depends on one throwing the ball skilfully, but it requires besides one to catch it dexterously, that the game may be gone through according to the rules for ball; so also is it the case that teaching is reliable when faith on the part of those who hear, being, so to speak, a sort of natural art, contributes to the process of learning. So also the earth co-operates, through its productive power, being fit for the sowing of the seed. For there is no good of the very best instruction without the exercise of the receptive faculty on the part of the learner, not even of prophecy, when there is the absence of docility on the part of those who hear. For dry twigs, being ready to receive the power of fire, are kindled with great ease; and the far-famed stone attracts steel through affinity, as the tear of the Succinum drags to itself twigs, and amber sets chaff in motion. And the substances attracted obey them, attracted by a subtle spirit, not as a cause, but as a concurring cause.