Saladin was saddened. ‘And is this why you want to build your engines? To change the way people look at you?’

She stared him down.

But if this was true, Saladin saw with dismay, then his mother had no choice but to pursue her dream of engines of war; the logic of her personality dictated it. And, Saladin sensed with dread, he was destined to follow her.

The door crashed open. They all flinched.

A monk burst in, tall, skinny, agitated, and with his tonsured hair comically sticking up around his bald pate. He looked younger than thirty. ‘Thomas!’ he shouted without preamble. ‘Good to see you again. And this must be Joan of Jerusalem, and her son, fascinating, fascinating, you who brought the conundrum of time to my door, you who believe past and future are all muddled up.’

Thomas said, ‘Roger—’

The man dumped a leather folder on a low table, and kept talking. ‘And why should time not be mixed up? All is in flux, the world is an unstable place. Heraclitus pointed out that he was never able to dunk his foot in the same river twice, for it changes with every instant - you see? So why, then, should we imagine that even the river of time is inviolable and unchanging? Perhaps it is more like the fabled Meander in Phrygia, which changes its course with every season, endlessly seeking the perfection of its Platonic ideal. So, then, perhaps history is made and remade, cutting through our lives as a wandering river cuts through sandbanks, for ever seeking some new and more perfect shape. Why not, I say, why not? Shall we get to work?’

Joan turned to Thomas. ‘Who is this person?’

‘One of the liveliest minds of this new age of scholarship, that’s who,’ said Thomas.

‘One of . . .’

‘Which is why I took your puzzle to him. Joan, Saladin, this is Roger Bacon, born in Ilchester, trained in Oxford, and now lecturing in Westminster. ’

‘Don’t forget Paris,’ Bacon put in.

‘I have been aware of his career since his student days - oh, a decade back now. You studied the classics at Oxford, did you not, Roger?’

‘And geometry, arithmetic, music, astronomy. I worked under Robert Grosseteste.’

‘The bishop of Lincoln—’

‘Who has led the reintroduction of the works of the Greeks into England.’

‘Roger lectured in Paris—’

‘I earned my master of arts degree there. I saw Alexander of Hales there, and twice saw William of Auvergne dispute . . .’

This fast-paced duologue was hugely confusing to Saladin, who had heard of none of these scholars.

‘I see myself now as a dominus experimentorum,’ Bacon said.

Joan glanced at Thomas. ‘What does that mean?’

‘One who studies the physical world,’ Thomas said, ‘and tinkers with it, in the hope of learning more about the truths of God.’