THE book is not a treatise on aIl cerebral mechanisms but a pro- poscd solution of a specific problem: the origin of the nervous system's unique ability to produce adaptive behaviour. The work has as basis the fact that the nervous system behaves adap- tively and the hypothesis that it is essentiaIly mechanistic; it proceeds on the assumption that these two data are not irrecon- cilable. It attempts to deduce from the observed facts what sort of a mechanism it must be that behaves so differently from any machinc made so far. Other proposed solutions have usuaIly left open the question whether so me different theory might not fit the facts equaIly weIl: I have attempted to deduce what is necessary, what properties the nervous system must have if it is to behave at once mechanisticaIly and adaptively. For the deduction to be rigorous, an adequately developed logic of mechanism is essential. Until recently, discussions of mechan- ism were carried on almost entirely in terms of so me particular embodiment-the mechanical, the electronic, the neuronie, and so on. Those days are past. There now exists a weIl-developed logic of pure mechanism, rigorous as geometry, and likely to play the same fundamental part, in our understanding of the complex systems of biology, that geometry does in astronomy. Only by the dcvelopment of this basic logic has thc work in this book been made possible.