We know how the details of our visual experience, like the experience of size constancy of objects, are related to our need to survive. But what is the evolutionary function of the experience of the ineffable and irreducible? Humphrey points to a feature of consciousness that has been surprisingly neglected. “The bottom line about how consciousness changes the human outlook — as deep an existential truth as anyone could ask for — is this: We do not want to be zombies,” he writes. “We like ‘being present,’ we like having it ‘be like something to be me.’ ”

Humphrey ingeniously works out the many consequences of this apparently simple fact. He points out, for example, that we humans will work as hard to get a newer or more vivid or more intense experience as we will to get a meal or a mate. Almost as soon as we could use tools to make hearths and spears, we also used them to construct consciousness-­expanding art installations in painted caves like Altamira. We fear death so profoundly not because it means the end of our body but because it means the end of our consciousness — better to be a spirit in heaven than a zombie on earth.

There is a story that Samuel Beckett was walking through the park with a friend, and exclaiming at the beauty of the day. “Yes,” said the friend, “it’s the sort of day that makes you feel good to be alive.” “Ah, now,” Beckett replied, “I wouldn’t go that far.” But most of us, most of the time, would go that far.

Humphrey argues that this is the result of a benign evolutionary illusion. It does feel good to be alive, and it feels especially good to be me being alive. And that in turn makes us go to great lengths to extend our lives and to fend off death. Human beings don’t do this just with the blind struggle of the hunting predator and the fleeing prey, but with elaborate long-term inventive planning. And that does help us extend life and hold off death.

Similarly, we are most vividly conscious of the unexpected and the novel — consciousness is linked to curiosity and exploration. So, Humphrey argues, the thirst for consciousness keeps us on the move, reveling in new information even when the immediate usefulness of that information isn’t apparent. In the long run, though, pursuing new information does give us important and distinctively human evolutionary advantages.

Just as the moon illusion is an effect of size constancy, the illusions of ineffability and irreducibility, in Humphrey’s view, are effects of our human capacity for self-reflection, long-term planning and innovation. The brain knows the real secret of seduction, more effective than even music and martinis. Just keep whispering, “Gee, you are really special” to that sack of water and protein that is a body and you can get it to do practically anything.

Humphrey’s ideas are appealing, but they aren’t always precise, and it will take a lot of empirical work to discover whether they are true. Evolutionary arguments have to go beyond just-so stories, and it’s not easy to see just how you could test Humphrey’s hypotheses. And even if we understood the evolutionary function of consciousness, we would still need to understand just how the brain accomplishes those functions.

While you can’t help sharing Humphrey’s own exuberant enthusiasm about consciousness, maybe gloomy old Beckett had a point too. We are, after all, as intensely and unbearably conscious of grief and pain as of joy and hope, and we humans seek out oblivion as well as exaltation. Still, you would do well to let Humphrey talk you into thinking that experience is meant for our delight — at least for an hour or two.