This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book explains the basic idea that the mental and the physical are identical, and focuses on some of the consequences that seem to follow from it. It shows how the prevailing, essentially Cartesian, orthodoxy gives rise to the central problem of the relation between two categorially distinct domains, the mental and the physical. The book also explains the main consequences of physical determinism for the traditional concept of moral responsibility and moral institutions. It describes the main outstanding objections to identity theory from considerations of logic, empirical science and epistemology. The book considers the principles used in law for determining criminal culpability, with special reference to the reasonable man criterion of provocation law and the objective test.