Callum Roberts is a professor of marine conservation at the University of York in Britain and the author of “The Ocean of Life: The Fate of Man and the Sea.”

"The two of you look at each other. This one is small, about the size of a tennis ball. You reach forward a hand and stretch out one finger, and one octopus arm slowly uncoils and comes out to touch you. The suckers grab your skin, and the hold is disconcertingly tight. Having attached the suckers, it tugs your finger, pulling you gently in. . . . Behind the arm, large round eyes watch you the whole time."

Encountering an octopus in the wild, as Peter Godfrey-Smith argues in his fascinating book, “Other Minds,” is as close as we will get to meeting an intelligent alien. The octopus and its near relatives — squid, cuttlefish and nautilus — belong to a vast and eclectic group of creatures that lack backbones, the invertebrates. Collectively known as cephalopods (head-footed), they are related to snails and clams, sharing with them the unfortunate characteristic of tasting wonderful. Don’t read this book, though, if you want to continue eating calamari with an untroubled conscience, for living cephalopods are smart, beautiful and possessed with extraordinary personalities.

Cephalopods certainly look alien, so it is hardly surprising that science fiction writers have hijacked their characteristics for imaginary aliens. An octopus has three hearts that pump blue-green blood around its body, using a copper- rather than iron-based carrier for oxygen. Its eight tentacles are covered in suckers that enable it to amble, probe and manipulate with great dexterity. When in danger or enraged, it can switch to jet propulsion, and if danger persists it can eject an ink cloud of confusion and darkness. An octopus has few hard body parts (beak and eyes only), making its body infinitely malleable. It can thread itself through a hole not much larger than the diameter of its eyeball.

Most cephalopods are active hunters that seek prey (crabs and other molluscs, mostly) in full view of potential predators. Here their soft and tasty bodies are a liability. To counter this vulnerability, evolution has produced the most expert shape-shifters in the animal kingdom. Cephalopods are the ultimate color-changers, able to match their backgrounds in an instant so perfectly that even a keen observer can lose them. As invisibility cloaks go, theirs are close to perfect. Cephalopod skin is like a pixelated video screen. The top layer contains tens of thousands of tiny pockets of three different colors that can be opened and closed at will to display bright reds, yellows, browns or other shades, depending on the palette a particular species has. Below them lies a layer of reflective cells that interacts with stacked plates to create iridescence. Underneath them is another reflective layer to bounce back incoming light.

Cephalopod skin patterns are no mere color-matching trick. They are capable of scintillating moving displays, flashing pulses of color like passing clouds, forking silver lighting or shimmering waves from a stone thrown into a pool. Cuttlefish can play two screens at once on their bodies, sending signals to entice a possible mate from one side and warning off competitors with a separate display on the other.

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?"

Cephalopod brains are certainly impressive. Anyone who has come face to face with an octopus will have sensed that something special lurks behind its cat-like eyes. The Roman writer Claudius Aelianus said of them in the 3rd century that “mischief and craft are plainly seen to be characteristics of this creature.” Captive octopuses give full rein to that mischievous temperament, learning to pop light bulbs with jetted water, block the outflow of their tanks so they overflow and recognize individual people, squirting those they dislike with water (or perhaps water jets are an aquatic sign of friendship?). With half a billion neurons to play with, there is space for a lot of complexity. Admittedly, that’s only about half a percent of humans’ hardware, but it compares favorably with dogs’.

There is an important difference in how octopuses use their neurons, echoing Godfrey-Smith’s earlier quip about an alien intelligence. There are more neurons in their tentacles than in their brains. This gives them exquisite sensitivity, allowing tentacles to touch, taste and even, to a point, see and think independently.

But how far can cephalopods take their mental power? Are they capable of conscious thought? Godfrey-Smith treks through some rather testing philosophical and psychological terrain to conclude in the negative. While cephalopods are capable of exceptional complexity in their signalling, the machinery of interpretation is too limited. Humans, perhaps uniquely, have gained the ability to step outside ourselves, to think about our thoughts by means of an unstoppable internal monologue. While cephalopods can produce highly patterned signals, they can’t see their own skins, Godfrey-Smith argues, so he rules out the possibility of any internal monologue.

And yet . . . could we be missing something here? While they can’t see much of their own kaleidoscopic skins, they can clearly sense inside what they are doing. Remote cameras on the seabed show octopuses crackling with color changes, even when there is no other creature present to observe them. Godfrey-Smith believes this is just a byproduct of neural activity, no more than an expressive quirk. But maybe it isn’t. Perhaps they are talking to themselves.

Interpreting what goes on in other minds is still beyond the capabilities of philosophy or science. After reading this book, to paraphrase Byron, you will “love not man the less, but cephalopods more.”

Other Minds
The Octopus, the Sea, and the Deep Origins of Consciousness

By Peter Godfrey-Smith

Farrar Straus and Giroux. 255 pp. $27