The Air Loom, if Matthews revealed its existence under questioning, would now be recognised immediately as a classic paranoid delusion. But in 1797 it was something that had never been encountered before, and would emerge as the baffling leitmotif of a case that was unprecedented in almost every imaginable way.

To look back before 1800 is to enter another world, one where the number of institutions for the mad was a tiny fraction of today's and what we would now call mental disorders were often understood as religious ecstasies or diabolical possessions.

Up to this point, it was rare for the mad to be distinguished from the poor, the homeless, the indigent, beggars, vagabonds, petty criminals and others who were unable to fit into society or take care of themselves. It was rare, too, that they were locked up.

The Bedlam that greeted James Tilly Matthews, then, was not so much a baroque spectacle of depravity as an exhausted and run-down public institution, its building falling apart and its professional image tarnished.

It was Matthews, of course, for whom the verdict was the greatest disaster. Not only had he failed to escape from Bedlam, but the anomalies of the case made it highly unlikely that he would have the chance to appeal again. His family and friends had assembled an impeccable case, most of which had been ignored.

The French army had crowned a campaign of extraordinary successes by defeating the Austrians at Jemappes and pressing on to occupy a large swathe of Belgium and threaten Holland. For Britain, this changed everything: a French republic that spread across the North Sea coast meant the entire coastline facing Britain would be in Republican hands.

But, mad or sane, Matthews was a man of no ordinary persistence. He was not prepared to renounce the peace plan, any more than he would be prepared to renounce his madness a few years later. A month later he was back in France, this time for an extended stay.The optimistic dawn of his revolutionary adventures was coming to an end, and his dark night of the soul was about to begin.

Come the revolution, however, mesmerism was reconceived once more. From its beginnings many had seen it as an aristocratic fad: Mesmer (by this stage long gone to Germany and Switzerland) had made a fortune from the nobility, charged the huge fee of 100 livres for admission to his Society of Universal Harmony, and even been offered a pension for life by Marie-Antoinette.

We are now edging across the boundary - always a porous one - between self-justification and fantasy. Matthews' story is by no means a complete fantasy: we can recognise every event. But the frame of reference is somehow shrinking, and momentous world events being rewritten around the actions of a minor player.