What constitutes consciousness or intelligence? This is a question that has proved to philosophers to be an intellectual dead-end. Now William Calvin, by looking closely at animal and human intelligence and a wide range of evolutionary evidence, has broken new ground that will help us understand mental illness and illuminate the whole notion of what it is to be a person. Calvin begins by asking what intelligence is. He moves to the Why of intelligence, where evidence from chimpanzees is important, before coming to the all-important How of intelligence, the cerebral codes and Darwinian processes that operate within seconds to produce intelligent thought and action.