Ibrahim snapped, 'Our family has lived in this den of decadence for too long. It has poisoned our blood, which must be cleansed!' And he stalked off, unsatisfied.

So Peter was left alone with this woman, her languid form draped on a divan. Impossible fantasies ran through his head.

Subh sighed. 'It is a trial to have a son whose soul is so much purer than mine. A reminder of the time when the holy Almohads ran all our lives, and the Almoravids before them.'

After the fall of Toledo a century and a half earlier, a bruised al-Andalus had fallen under the sway of cultists from across the strait, Almoravids and later Almohads, men of the desert with veiled faces who dressed in skins and stank of their camels, devout, disciplined and cruel. Attitudes hardened on both sides. The popes granted crusading indulgences to knights who fought in Spain, challenging the fundamentalism of the desert warriors.

'The boy means well, of course. But he simply isn't pragmatic. Are you pragmatic, scholar? Or are you religious?'

'Not especially, though I do plenty of work for the religious houses, mostly the Franciscans. My ambition is to be a philosopher, for which I need to find patrons – like yourself, to whom I am eternally grateful.' Peter's career was a new sort, unimagined not so long before. Thanks to the injection of scholarship from the conquered regions of al-Andalus there had been an explosion of learning across Christendom, and all over Europe itinerant scholars like Peter were trying to make a living. 'Of course,' he went on, 'the task of the scholar is to reconcile all our philosophy with the revealed truth of God.' That was the official truth, but actually, it seemed to Peter, thanks to the Aristotelian studies of the Moorish scholars, across Christendom the close ties of devotion and scholarship were loosening.

Subh was still thinking about her son. 'Ibrahim can't see that the subtext of the little encounter today is my rivalry with Alfonso the Fat, and the long war he is waging to destroy me. That's why he was trying to have poor Zawi put down – it's nothing to do with the law, or what Zawi may or may not have actually done. It's all to get at me.'

'Why would he wish to do that?'

'Because I work against him. He offends me, in his hypocrisy, his exploitation of the mudejars, and the size of his arse.'

The term 'mudejar', al-mudajjar, meaning literally 'the tamed', referred to Muslims still living in Cordoba, Toledo and other territories reconquered by the Christians. There was work for them to do, as clerks, accountants, lawyers. The more enlightened Christian rulers and wealthy folk even employed mudejar artisans to restore or rebuild their houses and palaces in a Moorish style.

'And in Cordoba,' Subh said, 'Alfonso has spent the six years since the city's conquest establishing himself as a middle man. Wealthy Christians find it easier to deal with a man who presents himself as a Christian, you see, than with the children of the desert – even though ten years ago "Alfonso" was as Moorish as an Almohad. So Alfonso sells Moorish work at the highest prices, while paying the Moors a pittance, and growing rich himself in the process. Why, he even exploits some of my own family. Can you believe that? He despises me, you see, because I stand up to him.'

'You were very brave to face down the crowd.'

'Brave for a woman, you mean?' She snorted. 'Well, I have to be strong, for all the men have fled. It was a bad time when Cordoba fell. My husband was already dead, killed fighting the Christians. Then King Fernando laid siege to the city. We capitulated; after six months we had no choice.' She paused, and her eyes were distant. 'Best not to speak of those times. That first evening a bishop entered the mosque to "cleanse" it, as they put it, and rededicate it to Christ. And they took away the bells of the church of Saint James, and returned them to Santiago de Compostela from where they had been stolen by al-Mansur, more than two centuries ago. Christians don't forget, or forgive! – but then,' she said fiercely, 'neither do Muslims. When the city gates were opened to the Christians, those who could afford to do so fled south, to Seville or Granada, even across the strait to the Maghrib.'

'Why did you not flee?'

'Because,' she said grandly, 'those left behind, the marginal and the poor, the toy-makers and the saddlers, the farmers and the potters, good Muslims stranded in a Christian city, have nobody left to stand up for them. And besides I am the descendant of a vizier. As you, in your rummaging in the libraries of Toledo, have proven for me, I hope.'

Her warm look stirred his blood again. But he paused, for he knew that what he had found in Toledo was more complicated than that.

She recognised his hesitation. 'Don't tease me, Peter of Toledo. Did you find what I wanted?'

'Yes. And no.'

She made a gesture like swatting a fly. 'A typical scholar's response – infuriating! Is that all I get for my money?'

'In the archives I explored in Toledo,' he said carefully, 'I found answers to your questions – and more. Some of this will please you. Some of it will not.'

'Tell me, then. Did you find my ancestor?'

'Yes.' In fact that part hadn't been hard. Even in the fractured age of the taifas, the Moors had always been fine record-keepers. 'Yes, there was a vizier; his existence isn't just a family tradition. And his name, as you know, was Ahmed Ibn Tufayl. I can prove a direct line of descent, in places through the female line, to you.'

'Hah! I knew it! Oh, every Muslim in Cordoba claims descent from one royal lineage or another. But I knew we were different – I knew it.'

'I have some details of his career' – and Peter patted his bag – 'but what may interest you more is not how Ibn Tufayl lived, but how he died.'

V

'Much of this was written down after his death by his granddaughter, Moraima, who survived him.'

'My distant grandmother,' Subh breathed, eyes heartbreakingly wide.

'Over a hundred years dead.'

Peter summarised for her the story of Sihtric, the priest from England. 'Who as you know,' he said cautiously, 'fathered Moraima by Ibn Tufayl's daughter.'

She sighed. 'We've learned to live with that, I think.'

'The question is why Sihtric came to Spain. He approached Ibn Tufayl for sponsorship for a project: the development of designs for marvellous weapons called the Engines of God.'

Subh's eyes widened at that. 'Weapons?'

'These designs had a mysterious origin,' Peter said. 'Or murky, you might say. They were supposed to be the result of visions, divine or satanic, implanted in the mind of an English monk, now two centuries dead, who-'

Subh waved her hand impatiently. 'I've no time for visions and miracles. Tell me about these weapons. Did Sihtric build them? What became of them?'

He told her how Sihtric, with great difficulty, had got as far as a few prototypes. 'But there was something missing, so Sihtric came to believe. An agent he called Incendium Dei – the Fire of God. It was something like Greek Fire, perhaps – though there I'm guessing. It was mentioned in the designs, but nothing the alchemists could crack.'

'And then what? Get on with it, man!'

'And then,' Peter said, 'a man called Orm Egilsson, English or possibly a Dane, came to Cordoba in search of Sihtric. He brought with him his son, called Robert. Orm had a head full of a prophecy of his own, a "Testament of Eadgyth", or Edith, something to do with a mysterious figure called the Dove. And he seems to have been determined to put a stop to Sihtric's work.'

'Why? No, don't answer that. Prophecies and visions! Sometimes I think the whole world is gripped by pious madness. Did he stop Sihtric?'

'In a way. Orm's arrival upset everything – in particular the relationship between Sihtric and Ibn Tufayl.'