This book is nothing if not ambitious for in it Humphrey makes the uncompromising claim that he has solved the problem of consciousness. His position is that the solution is essentially simple and even in a facetious sense boring. Fortunately, however, his book is not boring; it is written with his usual verve, humour, and above all clarity.
The main steps in his argument are conveniently summarized in a chapter near the end of the book. At the outset he makes a fundamental distinction between sensation and perception. Animals, he says, have evolved quite separate ways of representing what happens at the body surface. Sensations are representations of "what happens to me" and are associated with feelings (liking, disliking and so on), while perceptions are representations of "what is happening out there" and are not associated with feelings. And he sees consciousness as being connected with sensations alone. So, for Humphrey, consciousness is just "having sensations". Descartes got it wrong: instead of "I think, therefore I am" he should have said: "I feel, therefore I am".
Humphrey then goes on to analyse what "having sensations" means. Here he adopts an evolutionary approach, to show how present-day sensory activities could have developed step by step from primitive beginnings. First there would have been a simple "wriggle of acceptance or rejection" by an amoeba-like organism in response to stimulation at the body surface. Later this was mediated by nerves travelling from the body surface to the brain and back again; next, this loop was short-circuited by targeting the response on the incoming sensory nerve rather than the body surface, and finally reverberating feedback loops developed within the brain. This sequence of events, he believes, is plausible physiologically, clinically (in relation to the known effects of damage to the nervous system), and evolutionarily.
The sequence of steps just outlined seems intuitively plausible and it is hard to deny that something like it must have occurred. And Humphrey's insistence on the importance of feeling as opposed to intellect finds an echo in recent writing on neurophysiology; see, for example, Antonio Damasio's Descartes' Error. But, as Humphrey concedes, there will always be critics who object that his theory is mechanistic and unmysterious and does not really tell us what consciousness is. He imagines them reading his account and saying: "Is that all?" He admits, in fact, that at one time he was troubled by the same thought. Now, however, he finds that this was the wrong way of looking at the matter and the difficulty has evaporated for him.
At bottom he is putting forward an "identity theory". That is, he holds that consciousness just is the activity of the brain mechanisms he discusses. And he believes that it is impossible for any mechanism of this kind not to be conscious, because there is a necessary identity between brain activity and consciousness. If he is right it would be impossible for the hypothetical "zombies" (creatures otherwise exactly like us who nevertheless lack consciousness) postulated by some philosophers to really exist.
Some philosophers insist on the validity of what has been called the "hard question" about consciousness. That is, they see a big or even an unbridgeable explanatory gap between events occurring in the brain and our subjective experiences. Others, in contrast, regard this gap as unreal, and think that there is nothing that needs to be explained. Ultimately, this disagreement seems to be a question of psychology. Some people find that it is inconceivable that any kind of brain activity could be identical with consciousness, and others see it as self-evident. It is unlikely that those who dislike identity theories will be persuaded that Humphrey has solved the puzzle, but those who think that no real puzzle exists will probably find that he has offered a plausible account of how consciousness arose.