The man waited in unsympathetic silence.

‘I calculated the likely size of the nails,’ Luca said miserably. ‘I calculated the number of pieces that they might have been broken into. It didn’t add up. There are far too many relics for them all to come from one crucifixion. The Bible says a nail in each palm and one through the feet. That’s only three nails.’ Luca glanced at the dark face of his interrogator. ‘It’s not blasphemy to say this, I don’t think. The Bible itself says it clearly. Then, in addition, if you count the nails used in building the cross, there would be four at the central joint to hold the cross bar. That makes seven original nails. Only seven. Say each nail is about five inches long. That’s about thirty-five inches of nails used in the true cross. But there are thousands of relics. That’s not to say whether any nail or any fragment is genuine or not. It’s not for me to judge. But I can’t help but see that there are just too many nails for them all to come from one cross.’

Still the man said nothing.

‘It’s numbers,’ Luca said helplessly. ‘It’s how I think. I think about numbers – they interest me.’

‘You took it upon yourself to study this? And you took it upon yourself to decide that there are too many nails in churches around the world for them all to be true, for them all to come from the sacred cross?’

Luca dropped to his knees, knowing himself to be guilty. ‘I meant no wrong,’ he whispered upwards at the shadowy figure. ‘I just started wondering, and then I made the calculations, and then the abbot found my paper where I had written the calculations and—’ He broke off.

‘The abbot, quite rightly, accused you of heresy and forbidden studies, misquoting the Bible for your own purposes, reading without guidance, showing independence of thought, studying without permission, at the wrong time, studying forbidden books . . .’ the man continued, reading from the list. He looked at Luca: ‘Thinking for yourself. That’s the worst of it, isn’t it? You were sworn into an order with certain established beliefs and then you started thinking for yourself.’

Luca nodded. ‘I am sorry.’

‘The priesthood does not need men who think for themselves.’

‘I know,’ Luca said, very low.

‘You made a vow of obedience – that is a vow not to think for yourself.’

Luca bowed his head, waiting to hear his sentence.

The flame of the candles bobbed as somewhere outside a door opened and a cold draught blew through the rooms.

‘Always thought like this? With numbers?’

Luca nodded.

‘Any friends in the monastery? Have you discussed this with anyone?’

He shook his head. ‘I didn’t discuss this.’

The man looked at his notes. ‘You have a companion called Freize?’

Luca smiled for the first time. ‘He’s just the kitchen boy at the monastery,’ he said. ‘He took a liking to me as soon as I arrived, when I was just eleven. He was only twelve or thirteen himself. He made up his mind that I was too thin, he said I wouldn’t last the winter. He kept bringing me extra food. He’s just the spit lad really.’

‘You have no brother or sister?’

‘I am alone in the world.’

‘You miss your parents?’

‘I do.’

‘You are lonely?’ The way he said it sounded like yet another accusation.

‘I suppose so. I feel very alone, if that is the same thing.’

The man rested the black feather of the quill against his lips in thought. ‘Your parents . . .’ He returned to the first question of the interrogation. ‘They were quite old when you were born?’

‘Yes,’ Luca said, surprised. ‘Yes.’

‘People talked at the time, I understand. That such an old couple should suddenly give birth to a son, and such a handsome son, who grew to be such an exceptionally clever boy?’

‘It’s a small village,’ Luca said defensively. ‘People have nothing to do but gossip.’

‘But clearly, you are handsome. Clearly, you are clever. And yet they did not brag about you, or show you off. They kept you quietly at home.’

‘We were close,’ Luca replied. ‘We were a close small family. We troubled nobody else, we lived quietly, the three of us.’

‘Then why did they give you to the Church? Was it that they thought you would be safer inside the Church? That you were specially gifted? That you needed the Church’s protection?’

Luca, still on his knees, shuffled in discomfort. ‘I don’t know. I was a child: I was only eleven. I don’t know what they were thinking.’

The Inquisitor waited.

‘They wanted me to have the education of a priest,’ he said eventually. ‘My father—’ He paused at the thought of his beloved father, of his grey hair and his hard grip, of his tenderness to his funny quirky little son. ‘My father was very proud that I learned to read, that I taught myself about numbers. He couldn’t write or read himself, he thought it was a great talent. Then, when some gypsies came through the village, I learned their language.’

The man made a note. ‘You can speak languages?’

‘People remarked that I learned to speak Romany in a day. My father thought that I had a gift, a God-given gift. It’s not so uncommon,’ he tried to explain. ‘Freize, the spit boy, is good with animals, he can do anything with horses, he can ride anything. My father thought that I had a gift like that, only for studying. He wanted me to be more than a farmer. He wanted me to do better.’

The Inquisitor sat back in his chair as if he was weary of listening, as if he had heard more than enough. ‘You can get up.’

He looked at the paper with its few black ink notes as Luca scrambled to his feet. ‘Now I will answer the questions that will be in your mind. I am the spiritual commander of an Order appointed by the Holy Father, the Pope himself, and I answer to him for our work. You need not know my name nor the name of the Order. We have been commanded by Pope Nicholas V to explore the mysteries, the heresies and the sins, to explain them where possible, and defeat them where we can. We are making a map of the fears of the world, travelling outwards from Rome to the very ends of Christendom to discover what people are saying, what they are fearing, what they are fighting. We have to know where the Devil is walking through the world. The Holy Father knows that we are approaching the end of days.’

‘The end of days?’

‘When Christ comes again to judge the living, the dead, and the undead. You will have heard that the Ottomans have taken Constantinople, the heart of the Byzantine empire, the centre of the Church in the east?’

Luca crossed himself. The fall of the eastern capital of the Church to an unbeatable army of heretics and infidels was the most terrible thing that could have happened, an unimaginable disaster.

‘Next, the forces of darkness will come against Rome, and if Rome falls it will be the end of days – the end of the world. Our task is to defend Christendom, to defend Rome – in this world, and in the unseen world beyond.’

‘The unseen world?’

‘It is all around us,’ the man said flatly. ‘I see it, perhaps as clearly as you see numbers. And every year, every day, it presses more closely. People come to me with stories of showers of blood, of a dog that can smell out the plague, of witchcraft, of lights in the sky, of water that is wine. The end of days approaches and there are hundreds of manifestations of good and evil, miracles and heresies. A young man like you can perhaps tell me which of these are true, and which are false, which are the work of God and which of the Devil.’ He rose from his great wooden chair and pushed a fresh sheet of paper across the table to Luca. ‘See this?’

Luca looked at the marks on the paper. It was the writing of heretics, the Moors’ way of numbering. Luca had been taught as a child that one stroke of the pen meant one: I, two strokes meant two: II, and so on. But these were strange rounded shapes. He had seen them before, but the merchants in his village and the almoner at the monastery stubbornly refused to use them, clinging to the old ways.

‘This means one: 1, this two: 2, and this three: 3,’ the man said, the black feather tip of his quill pointing to the marks. ‘Put the 1 here, in this column, it means one, but put it here and this blank beside it and it means ten, or put it here and two blanks beside it, it means one hundred.’

Luca gaped. ‘The position of the number shows its value?’

‘Just so.’ The man pointed the plume of the black feather to the shape of the blank, like an elongated O, which filled the columns. His arm stretched from the sleeve of his robe and Luca looked from the O to the white skin of the man’s inner wrist. Tattooed on the inside of his arm, so that it almost appeared engraved on skin, Luca could just make out the head and twisted tail of a dragon, a design in red ink of a dragon coiled around on itself.

‘This is not just a blank, it is not just an O, it is what they call a zero. Look at the position of it – that means something. What if it meant something of itself?’

‘Does it mean a space?’ Luca said, looking at the paper again. ‘Does it mean: nothing?’

‘It is a number like any other,’ the man told him. ‘They have made a number from nothing. So they can calculate to nothing, and beyond.’

‘Beyond? Beyond nothing?’

The man pointed to another number: –10. ‘That is beyond nothing. That is ten places beyond nothing, that is the numbering of absence,’ he said.

Luca, with his mind whirling, reached out for the paper. But the man quietly drew it back towards him and placed his broad hand over it, keeping it from Luca like a prize he would have to win. The sleeve fell down over his wrist again, hiding the tattoo. ‘You know how they got to that sign, the number zero?’ he asked.