How does the brain produce adaptive behaviour? In attempting to answer the question, scientists have discovered two sets of facts and have had some difficulty in reconciling them. On the one hand the physiologists have shown in a variety of ways how closely the brain resembles a machine: in its dependence on chemical reactions, in its dependence on the integrity of anatomical paths, and in the precision and determinateness with which its component parts act on one another. On the other hand, the psychologists and biologists have confirmed with full objectivity the layman’s conviction that the living organism behaves typically in a purposeful and adaptive way. These two characteristics of the brain’s behaviour have proved difficult to reconcile, and some workers have gone so far as to declare them incompatible.