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There is an old joke waiting to happen in the premise of this book - a philosopher puts on a wetsuit - but Peter Godfrey-Smith plays it entirely straight, and the result is one of the stranger and more rewarding popular science books of recent years. Part evolutionary history, part underwater memoir, part philosophy of mind, Other Minds uses the octopus as a way of asking how intelligence could have evolved twice on the same planet, independently, and what that second experiment might mean for the way we think about thinking itself.

The central hook is genuinely startling. The last common ancestor between humans and octopuses lived around 600 million years ago and was probably a flattened, worm-like thing with nothing much going on in the way of a nervous system. Everything cephalopods became, they became on their own. Godfrey-Smith argues that looking at an octopus is the closest we are likely to get to meeting an intelligent alien, and he makes this claim feel less like marketing and more like a sober biological fact.

The best chapters sit squarely in the water. Godfrey-Smith has spent years diving off the east coast of Australia, observing a site he calls Octopolis, where octopuses - normally solitary and hermit-like - have formed something resembling a community. His descriptions of these animals are careful and unshowy, and all the more affecting for it. Octopuses taste with their arms, see with their skin, solve puzzles, play with novel objects, and recognise individual human keepers even when those keepers swap uniforms. Most of their neurons sit not in the brain but distributed through eight semi-autonomous limbs, which he memorably describes as jazz players improvising under the loose direction of a central conductor.

Where the book becomes more divisive is when it moves from the reef toward the lecture hall. Godfrey-Smith is a professional philosopher, and he is serious about the question of how subjective experience could have arisen from biology. Some of this material is superb - his synthesis of embodied, global-workspace, and higher-order theories is lucid and generous - but the consciousness chapters can also feel like they are reaching for conclusions the evidence will not quite support. There is a sense, at times, that mystery is being resolved too quickly, and that a book which earned its authority through patient observation becomes less certain-footed when it tries to generalise from it. Readers arriving for the animals may find the philosophy heavy going; readers arriving for the philosophy may find it under-defended.

Still, these are the complaints of a book aiming higher than most. Godfrey-Smith writes with unfussy elegance, and he handles genuinely difficult material - evolutionary biology, neuroscience, the hard problem of consciousness - without ever talking down to the reader or hiding behind jargon. The book rewards patience, and it leaves behind an oddly persistent afterimage: the feeling that the world is stranger, older, and more crowded with minds than one had quite allowed oneself to believe. It is not a tidy book, but it is a generous and quietly transformative one.