When they say that heat is merely the movement of certain globules and light the centrifugal force that we feel, we are amazed. What! Is pleasure nothing but a ballet of spirits? We had such a different conception of it, and these feelings seem so far removed from those with which we are comparing them. The feeling of fire, the warmth which affects us in quite a different way from touch, the reception of sound and light, all seem mysterious to us. And yet it is as straightforward as throwing a stone. It is true that the smallness of the spirits entering the pores touches other nerves, but they are still nerves.