Stephen Hawking knew how to separate the man from the myth. The man was playfully self-deprecating, aware that being played by Eddie Redmayne in The Theory of Everything made his younger self seem more handsome than he had ever been. The myth, or as Hawking puts it in the opening chapter of this book, “the stereotype of the disabled genius”, was that of a visionary, an oracle, something beyond human. Hawking was far too self-aware to ever believe it. “To my colleagues, I’m just another physicist,” he says.

That’s not actually true, of course. The mathematician Roger Penrose, who collaborated on some of Hawking’s greatest work, says Hawking was “extremely highly regarded” by his colleagues, largely because he made “many greatly impressive, sometimes revolutionary, contributions to the understanding of the physics and the geometry of the universe”.

Hawking sketches a few of those contributions here, in passages that are both lucid and mind-bending – and, lest we forget, a great and costly gift to the world. “It was effortful for him to communicate,” his daughter Lucy Hawking says in the book’s moving afterword, “but he made that effort.”

And the effort has paid off. Hawking’s description of the Casimir effect, where quantum uncertainty manifests as a force that is measurable in the laboratory, provides a moment of giddy wonder. His answers to the deep physics questions of what lies inside a black hole, whether there is intelligent life elsewhere in the cosmos and how the universe began are a compelling – if sometimes unavoidably confusing – read.

Such passages make it all the more disappointing when Hawking takes on the role of social commentator. The observations he makes about artificial intelligence are curiously prosaic: “AI can augment our existing intelligence to open up advances in every area of science and society. However, it will also bring dangers,” he says. He offers odd truisms such as, “If we can connect a human brain to the internet it will have all of Wikipedia as its resource.” And when it comes to answering the question of the future of humanity, he is strangely timid. Will we have genetic engineering of humans? Probably. “Of course, many people will say that genetic engineering on humans should be banned. But I rather doubt that they will be able to prevent it…” Does he think it’s a bad idea? Not exactly. “In a way, the human race needs to improve its mental and physical qualities if it is to deal with the increasingly complex world around it and meet new challenges such as space travel.”

Ah, the lure of the great beyond. Because of the likelihood of a nuclear confrontation or an environmental catastrophe, we should work out how to leave the planet and colonise space, Hawking reckons. “Spreading out,” he says, “may be the only thing that saves us from ourselves.” He concedes that move will involve abandoning the flora and fauna of Earth, but Hawking seems to believe that humans deserve more of a future than other species. Leaving all other life to fend for itself is something humanity will have to have “on our conscience”.

This is strange, since he has said elsewhere that “we will need to consider transporting several thousands of people, animals, plants, fungi, bacteria and insects”. And here we reach the slightly perplexing nature of this book. It was “in development” at the time of Hawking’s death, and has been pulled together from speeches, interviews and essays. It’s unclear where Hawking’s words end, and where the voice of colleagues, collaborators, family and the Stephen Hawking Estate begins.

This is perhaps why it feels incoherent at times. Each chapter is written as a stand-alone answer to a “big question”, with no reference to near-identical material that appears in other answers. As a result, the concept of quantum uncertainty is introduced several times, while DNA is introduced twice, with jarringly similar phrasings.

A little more time spent on the editing, and a little less spent in marketing meetings, would have created a better book. The publishers would have sold a lot of copies even without the slightly tawdry sales pitch. It is billed as “the final book from Stephen Hawking” and, like a luxury product, was made available for review only days before its global launch at London’s Science Museum. That said, the greatest physicist of our age appeared in TV ads for Specsavers, Go Compare and BT: the hawking of Stephen is not a new phenomenon, and was always one in which he was fully complicit.

And it was a means to an end. Hawking tells us, quite credibly, that he has always written primarily to share his enthusiasm for these topics, and that is the way to approach this book. Hawking would be the first to tell each reader that, as a human being – the cosmos made conscious – they are as extraordinary as he was. So read it – and do read it – in the spirit he would have meant it. Not as some last opportunity to glean profound truths from the oracle, but as one remarkable human being sharing his thoughts, hopes and fears with another.

Michael Brooks is the author of “The Quantum Astrologer’s Handbook” (Scribe)

Brief Answers to the Big Questions

Stephen Hawking

John Murray, 256pp, £14.99