According to the Harvard psychologist Daniel Gilbert, every psychologist must at some point in his or her career write a sentence of the form: "The human being is the only animal that ----".
It is not only psychologists who feel impelled to write "the Sentence", as Brian Christian calls these statements of human uniqueness. Versions of the Sentence can be found scattered throughout the writings of theologians and atheists, rationalists and existentialists, revolutionaries and reactionaries and every sort of social and cultural theorist.
The attributes that are supposed to make human beings special have varied. Often those who insist that we belong in a different category from other animals fix on consciousness as the definitive human attribute. But other animals are not insensate automata, whatever Descartes may have liked to believe, and while we have intellectual abilities that other species lack, this is a difference in degree, not kind.
As Christian writes, "The story of humans' sense of self is, you might say, the story of failed, debunked versions of the Sentence." But this is only the starting point of his inquiry, for it is not other animals that are now threatening our sense of self. Belief in human uniqueness is being challenged by machines that we have invented.
As they become increasingly proficient in performing intellectual tasks, computers are shrinking the range of activities that can be claimed to be exclusively human. The sense that we are special is diminishing, and there are many who fear it could eventually disappear altogether.
But could it? In this lively and thought-stirring reconsideration of the matter, Christian suggests that the rise of computers might enhance our understanding of what it is to be human. The opposite of a technophobe, he believes artificial intelligence can enrich human life - not least by illuminating what is distinctive about human intelligence.
Giving the reader a thorough grounding in the complex history of artificial intelligence along with entertaining digressions on video games, speed-dating and much else, The Most Human Human is an invaluable sourcebook on computing in modern-day life. It is also a personal narrative, however, in which Christian recounts his participation in the Loebner Prize.
The annual competition for this award uses the Turing test, named after the late Cambridge logician Alan Turing, who worked at Bletchley Park during the Second World War deciphering code with the use of a primitive computer called Colossus.
In a seminal paper of 1950, Turing forecast that, by the year 2000, computers would be able to trick at least 30 per cent of human judges that they were conversing with another human being. By the end of the century, he wrote, "one will be able to speak of computers thinking without expecting to be contradicted". By 2008, his prediction still had not been confirmed, but in that year the best program failed his test by just one vote.
Everything turned on the 2009 competition, which Christian entered. Together with several others, he was paired off with the best available artificial-intelligence programs and given the task of convincing a judge that the judge was conversing with a human being, not a thinking machine. The result was a resounding victory of human beings over computers.
Whereas in 2008 the computers had almost reached Turing's threshold, a year later not a single vote went their way. The prize for most human-like computer was awarded to David Levy, author of Love and Sex With Robots and a pioneer of computer chess. The most human human prize went to Brian Christian.
So, what human abilities did Christian exercise that the computers could not mimic? With a degree in computing and philosophy, he is also a poet, and summarises one of the book's most compelling insights when he writes: "If poetry represents the most expressive way of using a language, it might also, arguably, represent the most human."
The amazing proficiency that computers display in many contexts depends on their superior ability to think digitally, using information that has been broken down into discrete bits.
In contrast, what is distinctive of poetry - and, for that matter, of human language in general - is the vital role of context and allusion, which cannot be broken down into separate units of information.
Human conversations are not composed of a finite number of particular exchanges; they take place against a background of tacit understandings, which often make what is not spoken as important as what is said. That is one reason why artificial intelligence programs have failed to replicate the subtlety of natural languages.
Christian notes that the ever more pervasive role of computers in our lives risks thinning out these tacit understandings. In a change that he regrets, Facebook has replaced the box in which people described their favourite activities with a drop-down menu.
The assumption is that people can come to know one another by ticking a list. But what makes us individuals is not which of a limited set of activities we choose to engage in. When we describe the things we love to do we are telling more about ourselves than we know. By eliminating the option of entering our own description of our favourite activities, Facebook has emptied these activities of some of their meaning.
Yet Facebook is no less popular. For many, it seems, the loss does not matter. In fact, one of the attractions of a life that is mediated through computers may be just this loss of meaning. Computers have been immensely liberating in all kinds of ways, but one of these is in opening up the possibility of a life composed of a succession of individual bits of information. Part of the charm of the wired life is the freedom from meaning it promises.
Seeking an extreme form of this freedom, the futurist Ray Kurzweil and his fellow believers look forward to what Christian describes as "a kind of techno-rapture, where humans can upload their consciousnesses on to the internet and get assumed, if not bodily, then at least mentally, into an eternal, imperishable afterlife in the world of electricity".
What is most striking about this fantasy is not that the uploading it envisions is at present technologically impossible. It is that such an uploading would entail leaving behind much that makes us human. Believers in the coming techno-rapture may some day succeed in projecting phantom versions of their conscious selves into cyberspace. Even if this proves feasible, what survives will be only a cartoon version of the human individuals that once existed. But perhaps this is what these techno-gnostics really want: to cease to be human.
Christian is surely right in arguing that the rise of computers need not erode the human sense of self. Instead, the result may be to bring into clearer focus what it is that makes us different from machines. If history is any guide, however, human beings do not greatly cherish the features that truly make them what they are - finite creatures, with limited abilities.
Quite the contrary, people will do anything they can to escape from being what they are. So, a version of the Sentence still holds true: the human being is the only animal that refuses to be itself.
The Most Human Human: a Defence of Humanity in the Age of the Computer
Viking, 320pp, £18.99
John Gray is the New Statesman's lead book reviewer. His latest work of non-fiction is “The Immortalization Commission: Science and the Strange Quest to Defeat Death" (Allen Lane, £18.99)