The planning function of the nervous system, in the course of evolution, has culminated in the appearance of ideas, values, and pleasures - the unique manifestations of man's social living. Man, alone, can plan for the distant future and can experience the retrospective pleasures of achievement. Man, alone, can be happy. But man, alone, can be worried and anxious. Sherrington once said that posture accompanies movement as a shadow. I have come to believe that anxiety accompanies intellectual activity as its shadow and that the more we know of the nature of anxiety the more we will know of intellect.
- Howard Liddell, from "The Role of Vigilance in the Development of Animal Neurosis"