Probably the profoundest difference between human and animal needs is made by one piece of human awareness, one fact that is not present to animals, because it is never learned in any direct experience: that is our foreknowledge of death. The fact that we ourselves must die is not a simple and isolated fact. It is built on a wide survey of facts that discloses the structure of history as a succession of overlapping brief lives, the patterns of youth and age, growth and decline; and above all that, it is built on the logical insight that one's own life is a case in point. Only a creature that can think symbolically about life can conceive of its own death. Our knowledge of death is part of our knowledge of life.