This planned pact would have cost more than it was worth

It may seem strange that the most important question that arises from the revelations in Paddy Ashdown's diaries should be a hypothetical one, but it is the nature of this particular counter-factual beast that the "hows" and "whys" are much less intriguing than the "what-ifs". What if, in other words, the great Labour-Liberal Democrat coalition, with its mission to "modernise Britain", had actually been formed? What if Mr Ashdown and Menzies Campbell had been invited to sit at the Cabinet table? What if the "project" to bring the parties of the centre-left closer together had worked?

It may seem strange that the most important question that arises from the revelations in Paddy Ashdown's diaries should be a hypothetical one, but it is the nature of this particular counter-factual beast that the "hows" and "whys" are much less intriguing than the "what-ifs". What if, in other words, the great Labour-Liberal Democrat coalition, with its mission to "modernise Britain", had actually been formed? What if Mr Ashdown and Menzies Campbell had been invited to sit at the Cabinet table? What if the "project" to bring the parties of the centre-left closer together had worked?

The answers matter, for the Ashdown-Blair dialogue offers a tantalising glimpse of a very different style of government. For a start, it would have meant that the mood music on Europe would have been played in a markedly louder key. Peter Mandelson, Robin Cook and Stephen Byers would have found natural allies in their Liberal Democrat colleagues in trying to ensure that, whatever the formal position, the prospect of entry into the euro was publicly kept alive and privately pressed on an often equivocal prime minister.

To be clear, though, we cannot safely presume that the substance of the policy would have been radically different. The voices of the polls, the eurosceptic press and, above all, a stubborn Gordon Brown, would have probably still prevailed. We would not have joined the euro in 1999, as Mr Ashdown wished. But at least we would not have reached the dismal pass we are at today, when Mr Blair blithely rejects the euro and there is loose talk about him tacitly ruling out euro entry for the next parliament.

The tribal tendencies of Mr Brown would also have meant that any SBS-style raid on his "war chest" by Mr Ashdown to spend more on education would also have been resisted. And one cannot be sure either that Mr Straw, Lord Irvine, or Mr Blair for that matter, would have proved any more liberal-minded on issues such as asylum, the Lords and freedom of information, although some of their worst excesses might have been curbed. Proportional representation might not be very much closer, although the non-proportional reform of the alternative vote system might be.

So the answer to "what-if?" is "things might have been better, but maybe not much." Whether such marginal improvements would have been worth rupturing his party and emasculating another source of opposition in a parliament massively dominated by Labour is a more uncomfortable question, and one that Mr Ashdown has not yet adequately answered.