Why do we not exhaust the heritage of the ages, spiritual and material for our immediate pleasure, and let posterity go hang? So far as simple rationality is concerned, self-interest can advance no argument against the appetite of present possessors. Yet within some of us, a voice that is not the demand of self-interest or pure rationality says that we have no right to give ourselves enjoyment at the expense of our ancestors' memory and our descendants' prospects. We hold our present advantages only in trust.