He looked at the food before him, picking at a bit of cheese politely but without enthusiasm. Clearly he was used to better, even as a slave. 'Bread and cheese, then?'

'Food helps, if one can keep it down. But the true cure for a hangover was taught to me by a wise physician in Alexandria almost ten years ago - when I was about your age, I suspect, and no stranger to wine. It has served me well ever since. It was his theory, you see, that when one drank in excess, certain humours in the wine, instead of dissolving in the stomach, rose like foul vapours into the head, hardening the phlegm secreted by the brain, causing it to swell and become inflamed. These humours eventually disperse and the phlegm softens. This is why no one dies of a hangover, no matter how excruciating the pain.'

'Then time is the only cure, sir?'

'Except for a faster one: thought. The concentrated exercise of the mind. You see, thinking, according to my physician friend, takes place in the brain, lubricated by the secretion of phlegm. When the phlegm becomes polluted or hardened, the result is a headache. But the actual activity of thought produces fresh phlegm to soften and disperse the old; the more intently one thinks, the greater the production of phlegm. Therefore, intense concentration will speed along the natural recovery from a hangover by flushing the humours from the inflamed tissue and restoring the lubrication of the membranes.'

'I see.' Tiro looked dubious but impressed. 'The logic flows very naturally. Of course, one has to accept the starting premises, which cannot be proved.'

I sat back and crossed my arms, nibbling at a piece of crust. 'The proof is in the cure itself Already I'm feeling better, you see, having been called upon to explain the mechanics of this cure. And I suspect I shall be entirely cured in a few minutes, after I've explained what you've come for.'