A stimulus is given to the will when one deprives oneself of what one desires: then the will becomes conscious of itself, alive; it wonders why it should not have it. For instance, a person wants to have peaches, but at the same time he is very much attracted to the flower of the peach. He thinks the flower is beautiful, and then the idea comes: why not let it remain on the plant? That will make him decide not to pick it. This gives him a stimulus, because first desire wanted to take hold of it, then sense wanted to work with it; and as light comes from friction, so also does will come from friction.