Three books by Doctor Ashby constitute a logical and sequential development of his own thinking. At the same time they parallel a critically important development in our conception of the nervous system and its relationship to behavior and to psychological processes. In 1952, the first of the three was the original edition of this book. Its goal then was to reconcile the fact that although the nervous system is a constant mechanism it can nonetheless produce variable behavior. Ashby points out that it is this that differentiates the brain as a machine from any other machine.

Between the first edition and the present one, there have been other important developments in our knowledge of the brain and its action. These have involved an understanding of the role of reverberating circuits and of information theory. The latter led to the realization that the nervous system is not a mechanism which does