He might as well have called on pigs to fly or bankers to become nuns. Since 2008 there have been numerous efforts to harmonise global financial rules. And in some respects progress has been made. Common capital standards have been agreed under the umbrella of Basel III. A global regulatory body, the Financial Stability Board, has been created.

The unpalatable truth that no minister wants to acknowledge, however, is that almost every step towards co-ordination has been offset by a step away from it elsewhere. Just after Mr Lew’s appeal, for example, Deutsche Bank revealed that it would have to reduce its American assets by $100bn because US regulators are tightening capital rules for foreign banks. The UK has done something similar.

The US is also unilaterally introducing the Volcker rule, forcing banks to shed proprietary trading, while Europe is introducing rules for asset managers. Even the Basel III accord is implemented differently in different places.

Is there a solution? One option might be for regulators and bankers to borrow ideas from international divorce or property disputes; or, as Annelise Riles, a Cornell law professor suggests in a paper presented last week in New York, from the arcane field of “Conflicts of Laws.”

Conflicts specialists determine which national law should apply when fights straddle borders. They do not rule on the contents of laws, or the merits of any case, but merely decide which legal regime to apply, based on technical factors such as the original intention behind national laws.

Until now, nobody has ever asked these specialist lawyers to grapple with financial regulations; instead they usually just handle divorce or commercial deals. But Ms Riles thinks regulators should learn from some of their ideas. If the G20 were to adopt a “conflicts” mentality, it could drop the fantasy of co-ordination. Instead, it could accept the reality that different countries have different rules – and appoint technocratic, apolitical specialists to decide which laws apply if a conflict arises. “This provides a far more nuanced, sophisticated, and nevertheless manageable approach,” Ms Riles argues. And some regulators and lawyers agree.

At the Federal Reserve, for example, some lawyers talk privately of creating a “conflicts” database to enable regulators and bankers to track national variations in a transparent way. And some derivatives lawyers in London, such as Jeff Golden, have suggested creating an international court to handle contentious complex securities deals – in effect, an equivalent for the derivatives world of the International Criminal Court in The Hague.

In the near future, such radical ideas are unlikely to fly. National politicians would be loath to cede authority to a private law court on such a sensitive public policy matter as banking rules; and the risk of financial contagion makes governments unwilling to trust foreign regulatory authorities.

And yet some regulators do now appear to be privately conceding Ms Riles’ point – namely that co-ordination is an unworkable fantasy – and quietly hunting for alternatives.

Take the question of how to handle big bank failures. Five years ago, British and US regulators hoped to find a co-ordinated solution to this issue. But last year they tacitly recognised the inevitable – US and UK supervisors will never harmonise their approaches to shutting failed banks. Instead they agreed to respect each others’ rules, and came up with rough guidelines for deciding where each national regime applied. “This is about finding workable solutions,” says one former British regulator.

That is not nearly as good a solution as global harmonisation. But sometimes an imperfect answer is better than no answer at all. Or to put it another way, sooner or later, the G20 needs honestly to recognise that full co-ordination is a fantasy; and start talking about what other solutions could create a transparent and predictable financial world. And perhaps even talk to those divorce lawyers.