Arguments between philosophers about what constitutes consciousness or intelligence have proved to be an intellectual dead-end. In contrast, William Calvin, by looking at animal and human intelligence and a wide range of evolutionary evidence, has broken new ground that should help us understand mental illness and illuminate the whole notion of what it is to be a person. This text begins by asking what intelligence is. It then moves on to the "why" of intelligence and the syntax of intelligence - where evidence from chimpanzees is important - before coming to the "how" of intelligence, the cerebral codes and Darwinian processes that operate within seconds to produce intelligent thought and action.