How like the ancients are we? According to Ferdinand Mount, a good deal more than we might suppose. Given the idealised picture of the Greeks and Romans that many of us, in many respects, entertain, the initial response might well be one of scepticism. And, indeed, it is questionable whether Mount does much to change that during the course of this book. "It is," he says "as though the 1,500 years after the fall of Rome had been time out from traditional ways of being human." "In so many ways we are them" (the Greeks and Romans) "and they are us."

This seems fanciful. From Roman baths, via Turkish baths, to present-day health spa  yes, one buys that. Turkish baths were a direct descendant of Roman baths, perpetuated in the regions of the Eastern empire, brought here by Middle East enthusiasts in the 19th century, and, in their ritualistic format, a plausible prototype for the health spa. But gyms? An interest in food? A penchant for debate? Surely these are just healthy, commonsense institutions that may have lain dormant during two millennia of repressed, unfit, ill-fed Christianity, but whose resurgence now needn't make our world like the ancient one  certainly not in the significant and very conscious sense that 16th and 18th-century Europe was.

Perhaps because of this flaw in the central thesis there is much digression in each chapter into learned backwaters of whatever topic  fitness, sex, art  is being examined. It's true, for instance, that ancient Greece tolerated homosexuality, that this tolerance disappeared thereafter, and has only resurfaced now. This may make us like them, or more like them than subsequent eras, in one respect, but as to their "being us and us them", even here there are key differences. The Greeks accepted the "male" role in homosexual intercourse but ridiculed the "female" one. In The Laws, Plato condemns gay sex as unnatural. Conversely, at any rate in Dorian Greece, in certain times and states, gay sex seems to have enjoyed a quasi-institutional status for which there is no counterpart today.

In the section on food there are one or two striking parallels. We can relate to Cato, rolling up his sleeves and making his own panatone; there's a touch of Jilly Goolden in the poet Hermippus on wine  "I'm getting violets, I'm getting rose petals, I'm getting hyacinths." And the ancient Greek food critic Archestratus fore-echoes Rick Stein when he states that the only place to get your grey mullet is a swamp just south of Miletus. But even here, Mount overlooks a crucial difference. The dark and middle ages were doubtless a ghastly time to have to eat, but the unique blend of gourmet-ism and guilt that we in the 21st century have devised to torment ourselves is something the ancients would not merely have failed to relate to  they would have thought it insane. As for the dig at "my socialist friends" who are "as demanding as any hedge-funder when they sit down at a restaurant, as quick to complain if the service is slow and the food under- or over-cooked" . . . Where did that come from? One of Mrs Thatcher's speeches?

If the "we are them" claim seems far-fetched, there are some equally credibility-stretching assertions later. After quoting Pericles' famous speech outlining the virtues of the Athenian state, Mount says: "It has taken us two millennia and more to recapture the importance of those words", and as long to "recapture the spirit of scientific inquiry which animated the first philosophers".

I'd love to know which of the virtues of Periclean Athens Mount believes we, at the start of the 21st century  in many respects an age as leaden as Periclean Athens is held to have been golden  can be said to have recaptured. I suppose he means tolerance, but then what about the slightly later Athens that put Socrates to death? As for the spirit of scientific inquiry, if by "us" he means starting with Copernicus, Kepler, Galileo, how far back can the notion of "us" be made to go before it becomes meaningless? And does a mere unverified intuition on the part of Leucippus, Anaxagoras and Democritus that the world is made up of infinitesimally small particles really make them the forerunners of modern physics? Mount is in no doubt that it does.

He is equally unconvincing on religion. Fanatics such as Dawkins and Grayling (both of whom get quite a bashing) are doubtless fair game. With enemies like them, the religionists might say, who needs friends? But it is hard to see why a book purporting to be an objective account of a historical and cultural phenomenon should suddenly turn into an anti-atheist diatribe. This is in the same spirit as the dig at those restaurant-going socialists (he can't resist pointing out that ace God-basher Christopher Hitchens was once a Marxist) only writ large.

"A feature of human existence which has been so persistent across so many cultures for so many centuries surely deserves a careful examination." Why? Assuming religion is stupid, its having persisted, on a wide scale, for a long time, needn't make it any less so. This sort of argument plays into the hands of what Mount calls the "anti-God-botherers".

If he spent less time on his hobbyhorses and more on proving his thesis, this would be a better read. As it is, there is much to enjoy along the way from an author of obvious erudition, with a great flair for anecdote.

Ranjit Bolt is a playwright and translator, whose works include an adaptation of the Oedipus plays.