My love affair with octopuses began when I was 9. On a summer holiday by the sea, I found Octopus and Squid: The Soft Intelligence (1973) in my great-aunt’s bookcase. Written by Jacques-Yves Cousteau, the great pioneer of scuba diving, and his colleague Philippe Diolé, the book told of encounters between humans and cephalopods—the group that includes octopuses, squid, cuttlefish, and their more distant cousins, the nautiluses. A few days after I’d finished reading, I was out snorkeling and saw my first wild octopus. It was clambering over rocks in the shallows, changing color as it went. I was so excited that, after it vanished into a crevice, I leaped out of the water and began telling two strangers on the shore everything I’d learned from the book.

Cousteau and his team were the first to spend a lot of time—many hours at a stretch—in the water observing and filming wild octopuses and getting to know different individuals by visiting them regularly. Before long, some of the animals would come out to greet the divers, even climbing onto them and going for a ride. Others were shy, and would stay in their holes. Some appeared to develop preferences for particular humans. The divers wanted to know whether octopuses—as suspected—steal fish from fishermen’s nets, so they set up a net complete with several fish, and settled back to watch. Sure enough, an octopus came and helped itself to the lot. Another octopus opened a jar containing food, while a third seemed disturbed by its reflection when shown a mirror.

Cousteau’s accounts are anecdotes, not scientific experiments. Yet, taken together, they capture three aspects of octopuses—some species of them, at least—that strike anyone who spends time in the water with them.

First, different individuals have different temperaments. Some are shy, some are bold; some are inquisitive, some aggressive. Because of this individuality, people who hang out with them, whether in the sea, at a public aquarium, or in the laboratory, tend to give them names—an honor normally reserved for mammals such as dolphins and chimpanzees. Cousteau spoke of an octopus called Octopissimus; one scientific paper I read referred to Albert, Bertram, and Charles.

Second, some octopuses will engage with you. They might reach out an arm and touch your hand. They will investigate an object you present to them, giving every impression of thinking about it as they do so. All the while, they will appear to watch you with their large, mobile eyes. Again, these are behaviors we associate with dolphins and dogs—but not with, say, fish, let alone animals such as sea urchins or clams.

Third, octopuses often behave in surprising ways. Although Albert and Bertram were prepared to pull levers to receive pieces of fish, Charles destroyed the experimental equipment—he pulled it apart with his arms—and repeatedly squirted the experimenter with water. On a recent diving trip, my partner and I came across a little octopus sitting in the sand, two of its arms holding a large half clamshell over its head like a roof. For a while, we looked at it, and it looked at us. Then it shifted. It must have been reaching down with its other arms, because suddenly, like a small animated bulldozer, it tossed up a heap of sand. It did this several times, watching us closely and giving us the sense that, though it was interested in checking us out, it was also ready, if necessary, to pull the shell down like a lid and disappear into the seafloor.

The animals also frequently change their skin color and texture—which, to creatures such as ourselves, fine-tuned to watch faces for frowns and smiles, blushes and blanches, gives the appearance of emotional expressiveness. In other words, an encounter with an octopus can sometimes leave you with the strong feeling that you’ve encountered another mind.

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But that mind—if mind it is—has evolved along a route entirely different from the one that led to our own. The most-recent common ancestors of humans and octopuses lived about 600 million years ago, early in the evolution of animal life. Although much about our joint ancestors is obscure, they were probably small wormlike creatures that lived in the sea. This makes octopuses very different from other animals we suspect of sentience, such as dolphins and dogs, parrots and crows, which are much more closely related to us. In the words of Peter Godfrey-Smith, “If we can make contact with cephalopods as sentient beings, it is not because of a shared history, not because of kinship, but because evolution built minds twice over. This is probably the closest we will come to meeting an intelligent alien.”

Godfrey-Smith is a scuba-diving philosopher; his specialties are philosophy of biology and philosophy of mind. While out diving some years ago, he began encountering octopuses and cuttlefish, became intrigued, and started studying them. The result is Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness, a terrific mix of Cousteau-esque encounters with the animals in the wild (including a giant cuttlefish he calls Kandinsky), wide-ranging scientific discussion, and philosophical analysis. Beautifully written, thought-provoking, and bold, this book is the latest, and most closely argued, salvo in the debate over whether octopuses and other cephalopods are intelligent, sentient beings.

Mind, intelligence, sentience, consciousness—these are difficult, slippery terms, especially when applied to nonhuman animals. Cousteau remarked drily, “Scientists, although they concede that the octopus has a memory and that it learns quickly, do not use the word ‘intelligence’ in describing it.” He was writing in 1973, but it could have been yesterday. Several octopus researchers have told me that intelligence is a word they shy away from, either because of the SAT-like connotations, or because they feel that evidence for it is lacking, or because they think focusing on intelligence is narcissistic and fails to capture other important aspects of the wonder of these animals. Consciousness is even more contentious.

Arguably, though, it’s also narcissistic to assume up front that other animals are not, in some measure, intelligent or sentient, and that the human experience is unique in all respects. In any case, evolution doesn’t usually conjure complex traits from nothing; instead, they typically emerge from simpler antecedents. Light-sensing mechanisms run the gamut from molecules to eyespots to a huge variety of more complicated eyes. Nervous systems, too, show different levels of complexity; some are small and simple, while others are larger and more intricate. So why can’t the same be true of minds or consciousness? Indeed, as Godfrey-Smith reminds us, William James, the great 19th-century philosopher and one of the founders of psychology, argued that we should avoid assuming that human consciousness irrupted, fully formed, into the universe, and should seek simpler precursors. Taking this to its logical conclusion, Godfrey-Smith starts his quest for the origin of minds around the dawn of animal life, when nervous systems were first evolving into being.

But let’s get back to octopuses. In many ways, they are indeed profoundly alien. The animals are mollusks, and thus more closely related to other mollusks, such as clams and snails, than they are to any mammal. Most famously, they have eight arms, each lined with scores of suckers capable of grasping and tasting. Octopuses lack bones or an external shell (though they have a piece of cartilage that protects the brain). As a result, their bodies are soft, flexible, and stretchy—properties that allow them to vanish through tiny gaps. A small octopus can easily get inside an empty beer bottle. And in some species at least, the animals have an astonishing capacity for camouflage, instantly changing color, texture, and posture so as to blend in with lumps of coral on a reef or the blankness of the sand. This helps them hide from the many animals that fancy having octopus for lunch.

Then there’s the fact that they live in the sea, which means they operate in an entirely different sensory world—gravity doesn’t press, sound travels differently, and as the water gets deeper, the light becomes more and more blue before fading out altogether. This makes them, like many marine animals, hard to study in the wild. Just to find out what octopuses do all day takes tag teams of observers spending hours snorkeling or diving. Only a handful of groups have ever attempted such work. And octopuses have a reputation for being difficult to keep in the laboratory—they are sensitive to water quality, tricky to look after, and well-known escape artists.

Despite their “alien” credentials, however, octopuses do resemble us in some unexpected ways. Their eyes are remarkably like human eyes, an example of evolution converging on roughly the same solution from two wildly different starting points. (Octopuses don’t see in color, but because of the way their eyes are wired, they also don’t have a blind spot.) Like us, octopuses are dexterous, and can reach out and manipulate objects in the world. They display all those inquisitive, friendly behaviors reminiscent of dolphins and dogs.

Most telling of all, octopuses, along with cuttlefish and squid, have far larger, more complex nervous systems than any of their molluscan relations—or indeed, than any other invertebrates—do. The California sea slug (also a mollusk) has about 18,000 neurons, and honeybees, the invertebrate runners-up for neuron count, have roughly 1 million. The common octopus, Octopus vulgaris, has about 500 million neurons. This is more than five times the number in a hamster, and approaches the number in the common marmoset, a kind of monkey. (Humans have about 86 billion.) Going just on the basis of neuron count, you might think octopuses were a kind of mammal. But whereas mammals keep most of their neurons in their heads, an octopus’s nervous system is distributed throughout its body: About two-thirds of its neurons are not in its head, but in its arms.

Which raises several questions. What forces led octopuses to evolve such large nervous systems? Does having a large nervous system necessarily mean octopuses are intelligent, even conscious? And if they are, is their experience of consciousness something akin to our own, or is it—reflecting, perhaps, their distributed nervous system—entirely different?

Drawing on the work of other researchers, from primatologists to fellow octopologists and philosophers, Godfrey-Smith suggests two reasons for the large nervous system of the octopus. One has to do with its body. For an animal like a cat or a human, details of the skeleton dictate many of the motions the animal can make. You can’t roll your arm into a neat spiral from wrist to shoulder— your bones and joints get in the way. An octopus, having no skeleton, has no such constraint. It can, and frequently does, roll up some of its arms; or it can choose to make one (or several) of them stiff, creating an elbow. Surely the animal needs a huge number of neurons merely to be well coordinated when roaming about the reef.

At the same time, octopuses are versatile predators, eating a wide variety of food, from lobsters and shrimps to clams and fish. Octopuses that live in tide pools will occasionally leap out of the water to catch passing crabs; some even prey on incautious birds, grabbing them by the legs, pulling them underwater, and drowning them. Animals that evolve to tackle diverse kinds of food may tend to evolve larger brains than animals that always handle food in the same way (think of a frog catching insects).

But are they clever? Measuring intelligence in other animals is a challenge even when they’re not as remote from us as the octopus. And for octopuses, Godfrey-Smith observes, there is “a mismatch between the results of laboratory experiments on learning and intelligence, on one side, and a range of anecdotes and one-off reports on the other.” Yet as he points out, the very wealth of anecdotes is important information, showing as it does the flexible, unpredictable ways in which different individuals behave. While pigeons will spend hours pecking keys to get food rewards, octopuses are notoriously feisty. Charles is by no means alone in electing to squirt the experimenter instead of following the protocol.

As for assessing animal consciousness, that at first seems impossible. But one angle of attack is to work from the situation in humans. Over the past 30 years, a growing body of results has shown that conscious awareness represents just a fraction of what the human brain is registering. At the same time, scientists are identifying the type of tasks that do require consciousness. In particular: Consciousness seems essential for learning new skills—such as finding an alternative way home or opening a coconut. Taking up the work of the neuroscientist Stanislas Dehaene, Godfrey-Smith suggests that “there’s a particular style of processing—one that we use to deal especially with time, sequences, and novelty—that brings with it conscious awareness, while a lot of other quite complex activities do not.”

Like humans, octopuses learn new skills. In some species, individuals inhabit a den for only a week or so before moving on, so they are constantly learning routes through new environments. Similarly, the first time an octopus tackles a clam, say, it has to figure out how to open it—can it pull it apart, or would it be more effective to drill a hole? If consciousness is necessary for such tasks, then perhaps the octopus does have an awareness that in some ways resembles our own.

Perhaps, indeed, we should take the “mammalian” behaviors of octopuses at face value. If evolution can produce similar eyes through different routes, why not similar minds? Or perhaps, in wishing to find these animals like ourselves, what we are really revealing is our deep desire not to be alone.