(M17) When we survey the many different cases in which passing travellers are accustomed to add stones or sticks to existing piles, it seems difficult, if not impossible, to explain them all on one principle; different and even opposite motives appear, at least at first sight, to have operated in different cases to produce customs superficially alike. Sometimes the motive for throwing the stone is to ward off a dangerous spirit; sometimes it is to cast away an evil; sometimes it is to acquire a good. Yet, perhaps, if we could trace them back to their origin in the mind of primitive man, we might find that they all resolve themselves more or less exactly into the principle of the transference of evil. For to rid ourselves of an evil and to acquire a good are often merely opposite sides of one and the same operation; for example, a convalescent regains health in exactly the same proportion as he shakes off his malady. And though the practice of throwing stones at dangerous spirits especially at mischievous and malignant ghosts of the dead, appears to spring from a different motive, yet it may be questioned whether the difference is really as great to the savage as it seems to us. To primitive man the idea of spiritual and ghostly powers is still more indefinite than it is to his civilized brother: it fills him with a vague uneasiness and alarm; and this sentiment of dread and horror he, in accordance with his habitual modes of thought, conceives in a concrete form as something material which either surrounds and oppresses him like a fog, or has entered into and taken temporary possession of his body. In either case he imagines that he can rid himself of the uncanny thing by stripping it from his skin or wrenching it out of his body and transferring it to some material substance, whether a stick, a stone, or what not, which he can cast from him, and so, being eased of his burden, can hasten away from the dreadful spot with a lighter heart. Thus the throwing of the sticks or stones would be a form of ceremonial purification, which among primitive peoples is commonly conceived as a sort of physical rather than moral purgation, a mode of sweeping or scouring away the morbid matter by which the polluted person is supposed to be infected. This notion perhaps explains the rite of stone-throwing observed by pilgrims at Mecca; on the day of sacrifice every pilgrim has to cast seven stones on a cairn, and the rite is repeated thrice on the three following days. The traditional explanation of the custom is that Mohammed here drove away the devil with a shower of stones;(67) but the original idea may perhaps have been that the pilgrims cleanse themselves by transferring their ceremonial impurity to the stones which they fling on the heap.