Other Minds

Peter Godfrey-Smith, a philosopher of science and experienced scuba diver, blends evolutionary biology, philosophy of mind, and firsthand underwater observation to investigate how subjective experience arose in the natural world. Cephalopods, the group of mollusks that includes octopuses, cuttlefish, and squid, serve as his central case study because they evolved large brains and complex behavior on an entirely separate path from vertebrates, making them, as Godfrey-Smith argues, the closest thing to an intelligent alien that humans are likely to encounter.

The book opens with two encounters. In 2009, Matthew Lawrence, a scuba enthusiast on Australia's east coast, discovered a pile of thousands of scallop shells on the sea floor, with about a dozen octopuses living in excavated dens amid complicated coexistence at a site later named Octopolis. Earlier, Godfrey-Smith encountered a giant cuttlefish while snorkeling in Sydney: a three-foot-long animal hovering under a ledge, changing colors second by second. To understand such meetings, one must go back roughly 600 million years to the last common ancestor of mammals and cephalopods, probably a tiny worm-like creature. That ancestral population split: One line led to vertebrates and eventually to humans, the other to a vast range of invertebrates. Most invertebrates have small nervous systems, but cephalopods are the exception. Evolution, Godfrey-Smith contends, built complex minds at least twice.

Godfrey-Smith surveys the deep history of life. For billions of years, single-celled organisms dominated the sea. Even bacteria can sense their environments, and these capacities for signaling between organisms later became the basis for signaling within multicellular animals. The Ediacaran period, roughly 635 to 542 million years ago, offers the first fossil evidence of early animal life and appears to have been largely peaceful, with no evidence of predation. The Cambrian explosion, a rapid diversification of animal body plans beginning about 542 million years ago, changed everything as the invention of predation drove arms races favoring eyes, claws, and larger nervous systems. Only three major animal groups produced species with complex active bodies: arthropods, chordates such as vertebrates, and one group of mollusks, the cephalopods.

Godfrey-Smith traces cephalopod body evolution from early mollusks crawling under cap-shaped shells. Over time, these animals filled their shells with gas for buoyancy, rose into the water column, and became fearsome predators. Most were eventually outcompeted by fish. The nautilus survived among the ancient shelled forms, while other cephalopods abandoned their shells. The lineage split into an eight-armed group including octopuses and a ten-armed group including cuttlefish and squid. Octopuses lost their shell entirely, gaining extraordinary flexibility but also extreme vulnerability.

The book examines octopus intelligence through laboratory experiments and anecdotes. A common octopus has about 500 million neurons, comparable to some smaller mammals. In researcher Peter Dews's 1959 experiment at the Naples Zoological Station, an octopus named Charles broke the lever he was trained to pull for food, tried to haul the overhead light into his tank, and squirted water at experimenters. Octopuses in aquariums have learned to short-circuit lights, recognize individual human keepers, and time escape attempts for unwatched moments. Godfrey-Smith characterizes octopus intelligence as curious, flexible, and opportunistic, a non-social form distinct from the collective coordination of ants or honeybees. The octopus nervous system differs fundamentally from vertebrate designs: Rather than concentrating neurons in a centralized brain, the majority reside in the arms, with each sucker containing up to 10,000 neurons. Godfrey-Smith proposes that the body's open-ended possibilities created pressure for finer behavioral control, producing a feedback loop between bodily flexibility and nervous system expansion.

Godfrey-Smith addresses how subjective experience may have evolved, distinguishing between sentience, the basic fact that life feels like something, and consciousness, a more organized form of experience. A key factor is the evolution of feedback loops between sensing and acting: What an animal does affects what it senses next. He contrasts "latecomer" views, which hold that subjective experience requires sophisticated mechanisms found only in complex brains, with the "transformation" view he favors, which holds that basic forms of experience arose earlier and were reshaped by later additions. Evidence that zebrafish choose environments containing dissolved painkillers supports the idea that pain is a widespread form of experience predating complex cognition. The octopus's unusual brain-body relationship complicates any account of its inner life, since its arms are partly under central control and partly autonomous. Godfrey-Smith compares the central brain to a conductor whose musicians are jazz improvisers accepting only rough direction.

The book devotes attention to cephalopod color change. A cuttlefish's skin is a layered screen: Neurons operate muscles that pull millions of tiny sacs of pigment called chromatophores into different shapes, while reflecting cells beneath produce additional colors. Godfrey-Smith describes distinctive individuals like Matisse, who would explode from dark red into canary yellow, and Kandinsky, whose displays were the most extravagant Godfrey-Smith encountered. Paradoxically, cephalopods appear color-blind, possessing only one type of photoreceptor. Godfrey-Smith argues that some color changes go beyond camouflage or signaling, representing inadvertent expression of internal processes. He contrasts this with baboon communication as studied by researchers Dorothy Cheney and Robert Seyfarth: Baboons produce only a few calls but interpret them with great sophistication, while cephalopods have immensely rich expressive capacity but simpler social lives.

Godfrey-Smith examines the role of inner speech in human consciousness, proposing that it originates from efference copies, internal predictions the brain makes of its own outputs. When we speak, the brain generates an image of the intended sounds, and these auditory images eventually take on additional roles, forming unspoken sentences that organize ideas and exert self-control. This inner broadcasting provides a mechanism for what neuroscientist Bernard Baars called the global workspace theory of consciousness, which holds that conscious experience involves information broadcast widely across the brain. Cephalopod color displays lack such feedback loops because the animal cannot see its own skin patterns the way a person hears their own words, placing cephalopods on a different cognitive path.

The book confronts the paradox of cephalopods' short lifespans. Giant cuttlefish and most octopuses live only one or two years despite their large brains. Godfrey-Smith explains this through the evolutionary theory of aging developed by immunologist Peter Medawar and biologist George Williams: Harmful mutations acting late in life face almost no selection pressure, and mutations beneficial early but harmful later are actively favored. The loss of the protective shell gave cephalopods mobility and nervous complexity but made them extremely vulnerable to predators, compressing their lifespans. A deep-sea octopus observed by a Monterey Bay research unit brooded eggs for four and a half years in conditions of low predation, supporting the prediction that reduced external danger allows longer lifespans.

In the final chapter, Godfrey-Smith returns to Octopolis. Unmanned cameras and biologist David Scheel's analysis revealed that dark skin color predicts aggressive intent among the octopuses, while pale displays signal unwillingness to fight. Godfrey-Smith proposes the site originated when a metal object dropped on the sandy floor provided a rare safe den; octopuses brought scallop shells and left debris that accumulated into an artificial reef. Genetic evidence has backdated the split between octopuses and cuttlefish/squid to about 270 million years ago, making it likely that large nervous systems evolved at least twice within cephalopods. The 2015 octopus genome sequencing and researcher Christelle Jozet-Alves's demonstration of episodic-like memory in cuttlefish provide further evidence of convergent cognitive evolution.

Godfrey-Smith closes by reflecting that the mind evolved in the sea, where all early stages of life and nervous systems originated. He warns that oceans face enormous threats from overfishing and acidification, and ends with the observation that diving into the sea is diving into the origin of us all.