Letter 90 — On the Part Played by Philosophy in the Progress of Man (§23)

“Following this pattern,” he goes on, “someone placed two rough stones, the one above the other, in imitation of the teeth, one set of which is stationary and awaits the motion of the other set. Then, by the rubbing of the one stone against the other, the grain is crushed and brought back again and again, until by frequent rubbing it is reduced to powder.”