In the animal there is an uninterrupted chain of reactions starting with a stimulus, like hunger, and ending with a more or less strictly determined course of action, which does away with the tension created by the stimulus. In man that chain is interrupted. The stimulus is there but the kind of satisfaction is “open,” that is, he must choose between different courses of action. Instead of a predetermined instinctive action, man has to weigh possible courses of action in his mind; he starts to think.
