‘We must make shift to do the best we can,’ my father explained to me.

In Portugal we had passed for New Christians, Catholics, loyal to the Pope of Rome. Here in London we were Protestant refugees, like the Huguenots who had flooded into England since the bloody massacres in France. A hunted creature will take on any colouring it can to escape death.

Yet is it any wonder that I questioned the existence of God as I stood among these men, murmuring and swaying to prayers which seemed to me empty of meaning? They call us Marranos, we who change our faith as we change our country, or seem to do so. It is an old Spanish word which means ‘pig’, an insult whose significance is not lost on us. We think of ourselves as Anusim, the ‘Forced Ones’, driven under duress to take whatever is the dominant religion in the country where we live. It is only thus that we can survive.

That Sabbath day I felt more than usually uneasy all through the service. Perhaps it was the brief conversation with Sara about the man at the Marshalsea, the man Poley. For some reason I could not shake off the thought of him.

As we came outside into the bitter air, I was eager to be away. We do not linger long after service, talking to our friends and neighbours as you will see the Christians do after a church service, for it is better for us not to be seen gathered together in large numbers. Indeed, when we attended church at St Bartholomew’s, my father and I, as we must in order to obey the law, we behaved like the good Protestants we pretended to be, exchanging news and gossip with our neighbours – the butchers of Smithfield and the bakers of Pie Lane and the women who look after the daily needs of the patients at the hospital.

Now we turned to walk away from Mark Lane a little behind the Lopez family, who were making their way somewhat slowly through a group of men emerging from an inn, well-dressed men, gentlemen indeed. Young and full of high spirits, and genial with wine, calling out that they had just won a bet on a horse race. I saw Dr Lopez glance at them sharply before looking away, so that I followed his glance.

There in the midst of the group, with his arm round the shoulders of a handsome youth with bright eyes and a gallant air, was the man Poley. At first I thought I had conjured him up out of my own imaginings. How could that sick prisoner from the Marshalsea, though certainly rich, be here in easy company with such gentlemen as these?

‘Ah, Robyn, sweet Robyn!’ the handsome young man cried. ‘You shall dice with me for the colt, now that he has won his race. Come back to my lodgings, and we shall see which of us is the man that fair and fickle Dame Fortune shines upon.’

He stumbled somewhat over this speech, for he was more than a little drunk.

‘Why, Anthony,’ said Poley affably, ‘you know that you are always the winner.’ Then, as if my own eyes compelled him, he looked across the narrow street and met them with a bold stare. He smiled. Not at me, but to himself, secretly. And I averted my face and hurried after my father.

‘Ah, good morning to you, Kit. Have you worked those problems I set you?’

My mathematics tutor, Thomas Harriot, peered at me cheerfully through an untidy mop of hair. He constantly ran his hands through it, in despair at his own or his students’ inability to follow the crystal clear steps of logic to a perfect solution. And when success was achieved, he would push his hair up from behind, so that it stood on end like the crest of some exotic bird brought to England by one of the Queen’s sea captains.

‘I have,’ I said, hooking out a stool with my foot and sitting down opposite him. ‘I think I have established the correct proofs. The third one took me some time.’

He grinned mischievously.

‘That was meant to test you. And you solved it? Excellent, excellent!’

He took the sheets of paper from me and ran his eye over my careful workings. When his hand pushed up that crest of hair, I knew my solutions met with approval. Four years ago, when we had first arrived in London, my father used some of his small salary to provide tutors for me in music, mathematics and philosophy, while training me in natural philosophy and medicine himself. Now that I was working with him at St Bartholomew’s, the only studies I kept up were in mathematics. Two years ago I had been taken on by Harriot, a considerable honour, for he was recognised as the most gifted mathematician in England, though he had enemies. I suppose there will always be people who find mathematics a mystery akin to magic, and are suspicious of those who practice it.

‘Well done, Kit. I was not sure whether I was asking too much of you. Look, I have just acquired this new work on optics. You read Italian, do you not? Optics is throwing up some interesting mathematical problems. You may borrow it, if you like.’

‘Oh, yes!’ I was like a fledging bird, its mouth gaping. New knowledge, scholarship, was food for my hungry mind. I would never be able to attend university, so I gobbled up whatever scraps fell my way. I ran my fingers lovingly over the book, a squat little volume bound in dark green leather.

For the next hour, Harriot and I discussed the theories in the book and also worked on some problems in three-dimensional geometry. I had become interested recently in how artists can best represent three-dimensional objects on the flat plane of their canvases, and whether there might be useful mathematical solutions to the problem. My sessions with Harriot were a constant joy to me, refreshing as cool water in the desert. Here I had no need to think about the deceits of my life or the conflict of my beliefs. Here nothing mattered but the pure mind, separate from the distractions of body and soul. I went home whistling, with the precious book on optics tucked into my satchel. I had intended to discuss with my teacher the mathematics of musical harmony, but the thought of a precious new book to read had put it quite out of my mind.

For the next few days I forgot the man Poley, glimpsed so unexpectedly in the street, forgot even the fascination of optics, for there was an outbreak of the bloody flux in the western outskirts of London. I laboured with my father from dawn until late into the night, administering medicines to strengthen the gut. The sickness spread to our patients in the hospital, and to those who came in from the countryside to wait patiently in the old abbey cloisters until we could attend to them. The flagstones in the cloister were puddled with vomit and liquid, bloodstained faeces amongst the churned snow. Twice a day, one of the hospital servants threw a bucket of water over them, but the stench remained.

‘We must make up more of the tincture to heal interior bleeding, Kit.’ My father was heavy-eyed after three long days of labour in the wards. ‘If you will pound the herbs I will make up the mixture.’

I nodded. I caught a glimpse of myself reflected in the base of a copper pan which hung beside the hearth. Even in its distorted image I could see that my own face was drawn with weariness. Before we had returned late from the hospital, Joan had gone to her bed, leaving a pot of soup on a trivet near the fire and on the table, covered with a cloth to keep off the flies, a slab of pie fetched from the pie man round the corner.

‘I think we should eat first, Father.’

I pulled out his carved chair and beckoned.

‘Oh, very well.’ He gave a laugh which was part sigh. ‘We are both so tired we may make a mistake. Is the soup hot?’

‘It soon will be.’

I hung the pot on its hook and swung it over the fire, then threw on another log. The fire spat out a shower of sparks, but began to burn up brightly and I held out my hands to it. Walking home through the snow I had become more chilled than I realised. My boots were sodden and I prised them off with stiff fingers. Soon we were both spooning up the soup eagerly. Joan was not a particularly skilled cook, but the soup was substantial and warming, flavoured with a mutton bone and thick with carrots and onions. Between us we finished the entire pot.