Fascinating exploration of the phenomenon of intelligence - mostly human intelligence, although one of the chapters has a convincing argument that primates and animals such as dolphins have sufficient capacity to warrant the same legal protections as humans. It is, of course, an evolutionary argument, and one which Sagan applies to all the hominins (as much as he can, at least, for physical evidence is hardly exhaustive in some cases). The complex interaction of biology and environment has had a profound effect on our ability to think, and in consequence our bodies have developed some very strange characteristics. For instance, the left-hand/right-hand sides of the brain and how they process sensory information is delightfully weird, and the experiments and case studies which Sagan uses to help explain the science behind the history of the brain made the book clear and accessible.