The triad which began with The Sleepwalkers and The Act of Creation is completed with this very able and quite plausible (if sometimes sloppy) synthesis of new approaches to the structure and genesis of human beings. Koestler used his views on scientific and artistic modes of thought to elaborate his arguments about the human mind itself. He rejects the crude Cartesian mind-and-matter dualism signified by the title; but the book makes a lengthy attack on equally crude Behaviorists who reacted by denying altogether the fact of ""consciousness,"" a denial which transformed philosophy and psychology. The book is organized with reference to a thesis and a concept. The concept: ""hierarchical organization,"" which Koestler applies fruitfully to perception, memory, language, thought, and (more dubiously) to social structures. The thesis: that the ""old"" cortex and the ""new"" one function dissonantly--whence our pernicious intellectual-emotional split and our ""paranoid streak."" By way of a cure, Koestler vaguely outlines biochemical aids to reunion (a far cry, he adds, from LSD and 1984 pills). His sleek style, deft excerpts from prominent scientists, and his generous use of examples compound the book's attraction. Attractive it will be to a range of readers, from armchair philosophers to freshman biologists.