Man differs from other animals in many ways. One of these is, that he is willing to engage in activities that are unpleasant in themselves, because they are means to ends that he desires. Animals do things that, from the point of view of the biologist, seem to be labour for a purpose: birds build nests, and beavers build dams. But they do these things from instinct, because they have an impulse to do them, and not because they perceive that they are useful. They do not practise self-control or prudence or foresight or restraint of impulses by the will. Human beings do all these things. When they do more of them than human nature can endure, they suffer a psychological penalty. Part of this penalty is unavoidable in a civilised way of life, but much of it is unnecessary, and could be removed by a different type of social organisation.