AS the authors explain in the first line of their acknowledgments, they've established a bit of a franchise in this kind of thing. Their two previous books focused on, firstly, a row between Ludwig Wittgenstein and Karl Popper, their second on the battle between Bobby Fischer and Boris Spassky to become world chess champion. "We seem to be specialising in knock. . . down, dragout clashes between men of titanic gifts." They don't explain precisely why these bruising punch-ups fascinate them. Is it simply a curiosity with the old testosterone thing or is there some wider point to be made?

In 1776 David Hume, that great and influential British philosopher, brought Jean Jacques Rousseau, that great and influential Swiss philosopher, over to England.

Rousseau desperately needed to leave continental Europe. His radical views expressed with such eloquence and persuasiveness made him one of the most notorious intellectuals of the age. They also made him many powerful enemies.

Hume's rescue of his beleaguered colleague was an act of generosity typical of the man. Some warned him of Rousseau's quarrelsome reputation, the ease with which he took offence, but Hume's confidence in his own serene temperament made him discount this well-meaning advice.

It started well; Hume enjoying the role of generous patron and Rousseau effusively grateful. But of course it all ended in tears. Within 18 months the bitter dispute between these erstwhile friends was the hot gossip of drawing rooms throughout Europe.

The early part of the book outlines the personal history and cultural importance of the two protagonists.

This is well done. The details of Rousseau's life still have the ability to shock. A patron saint of the later French revolution, a passionate advocate of equality, he refused for 23 years to marry his servant mistress. They had five children and every one of them was taken to the Foundling Hospital in Paris and abandoned there. This didn't prevent him from subsequent pontifications on the subject of child-rearing.

Hume is a more staid figure, his private life almost monumentally dull. The implications of his ideas though, with its emphasis on critical thinking and scarcely veiled atheism, were at least as much a potential challenge to authority as the more flamboyant Rousseau.

So what was the substance of this volcanic, titanic row? Some passionately held principle, some abstruse intellectual position, a political argument? None of the above.

And that, in essence, is the problem with the book. It was just a fairly mundane clash of personalities, fuelled by Rousseau's paranoia and an uncharacteristic up rush of bad temper from Hume. As we inch through the incidents of the quarrel in which who wrote what to whom is told in painstaking detail, one admires the scholarship of the authors but really it becomes a little tedious. No fault line in Enlightenment thinking is exposed, no startling psychological insight into the main protagonists offered. It was just a nasty little row that did nothing to enhance the reputation of either man. It raged for a while and then fizzled out, altering little in the course of their lives or indeed anyone else's.

The book is well-written and the research clearly exhaustive but it's not enough. It may have been the authors' intention to lead us into an interesting byway of history, but this reader at least felt as though she'd ended up in a cul de sac.