Philosophers devoted much of their time and energy to the difficult task of proving the obvious. That they succeeded well is evident, since even physicians are more familiar with Descartes' Cogito, ergo sum than they are with his pioneer experiments on optics and vision. Old age, too, is obvious, but its definition becomes elusive. It begins in youth and progresses at an uneven pace in various organs of the body. Certainly it is physiologic rather than chronologic, and its attendant loss of physical and mental resilience involves a process of breaking rather than of bending. One is never quite sure when a person has died of old age, but one knows when he has died of disease. As causes of illness and death in old age, diseases of thoracic and abdominal viscera and of blood vessels are strikingly frequent, but as causes of senile decadence, diseases of the nervous system,