The cleft in the tree of life that separated the lineages that led to vertebrates and invertebrates happened 600 million years ago. One path, as Godfrey-Smith explains, led to progressively more complex intelligences, in the form of fish, reptiles, birds and mammals. The other path, while producing sophisticated means of sensing and surviving in a dangerous world, eschewed mental complexity. The cephalopods represent an island of intelligence in this invertebrate sea. They represent a separate experiment in the evolution of the mind.
Godfrey-Smith is not a scientist but a philosopher. This is, he says, a philosophy book as well as a scientific one. The question with which he wrestles is that of consciousness: “Does it feel like something to be one of the large-brained cephalopods, or are they just biochemical machines for which all is dark inside?”