The "contraries" are the anvil and the hammer which slowly forge souls by producing what might be called sensation in general, and sensation is a fertile cause of suffering each time the vehicles of consciousness receive vibrations that greatly exceed their fundamental capacity of sensation. Without sensation however--consequently without suffering--the body could neither walk,[19] nor see, nor hear, nor show any disturbance brought to bear upon it; there would exist no possible relation between the Universe and the "I," between the All and the parts, between bodies and souls; there would be no consciousness, or sensation of being, since no vibration from without would find an echo in the incarnated "centres" of life; no knowledge would be possible; man would be, as it were, in a state of nothingness; and, without suspecting it, his body might at any moment be crushed to the ground by the forces of Nature. But these material necessities are not by any means the only ones that demand sensation; without it, one of the principal objects of evolution--the development of "Egos"--would be impossible. As an example borrowed from the domain of physical sensation, we need only call to memory a well-known experience in childhood. All who have been at a boarding school know how heavy and fetid is the atmosphere of a dormitory in the early winter morning, when fifty boys have been breathing the same air again and again during the whole of the night. And yet, who suspected this until he had gone out for a few minutes and then returned to the bed-room? It needed the "contrary," the pure outside air, to make known the state of the atmosphere inside. The contrast produced sensation--that nauseous, suffocating impression of foul, mephitic air; suffering[20] generated knowledge of the vitiated air; as the result of this influence, the "centre of consciousness" felt itself an "I" distinct from its surroundings, and its "self-consciousness" received a slight increase. What might be called passional sensibility--desire, emotion, impulse--is, like physical sensation, another indispensable factor in evolution; it is the special element in the development of the animal kingdom as well as of the less evolved portion of the human kingdom.