Man differs from the lower animals because he preserves his past experiences. What happened in the past is lived again in memory. About what goes on today hangs a cloud of thoughts concerning similar things undergone in bygone days. With the animals, an experience perishes as it happens, and each new doing or suffering stands alone. But man lives in a world where each occurrence is charged with echoes and reminiscences of what has gone before, where each event is a reminder of other things. Hence he lives not, like the beasts of the field, in a world of merely physical things but in a world of signs and symbols. A stone is not merely hard, a thing into which one bumps; but it is a monument of a deceased ancestor. A flame is not merely something which warms or burns, but is a symbol of the enduring life of the household, of the abiding source of cheer, nourishment and shelter to which man returns from his casual wanderings.