He led me out into the courtyard, and past a group of buildings that contained the inmates. 'This is for the non-violent ones,' he said as we strolled in the warmth. 'The more difficult characters we keep in the block you can see over there. Alas, they get much less generous treatment; we don't have the money to do much for hopeless cases. There's no point, either. We can merely stop them harming themselves and others. In here.'

It was quite a pleasant surprise; I had imagined something like a Piranesi print, or Hogarth at his most despondent, but the room was light and airy, simply furnished and comfortable. Only the shadow of a large cross on the wall, where a crucifix had once hung, hinted at the building's previous purpose. There was one solitary person in it.

Signor Casanova – there was no other name to give him and in fact Marangoni never did find out who he was – was sitting in a corner, by a large window that looked out towards the Lido. He was reading a book, his head bowed, but was undoubtedly the man I had seen singing on the canal on my first night in Venice. Only the clothes were different; the hospital had taken away his old-fashioned costume, and clothed him in its drab, colourless uniform. It diminished him, that garb, made him seem less of a person. Certainly less disconcerting.

'Signor Casanova,' Marangoni called. 'A visitor for you.'

'Please be seated, sir,' he said, as though about to offer me a drink in his salon. 'As you see, I am well able to pass some time with you.'

'I'm glad of that,' I replied, as courteously as he. Already I had entered a sort of dream world; only later did it seem strange that I talked with such respect to a man who was insane, penniless, without even a name of his own. He set the tone of the conversation; I followed him.

He waited for me to begin, smiling benignly at me as I sat down opposite him. 'And how are you?' I asked.

'Very well, considering my circumstances,' he replied. 'I do not like being locked up, but it is hardly the first time. I was locked in the Doge's dungeons once and I escaped from there. I have no doubt I will leave here soon enough as well.'

'Really? And this was?'

'In 1756,' he said. 'I was accused of occult knowledge and of spying. A strange combination, I thought. But then authorities have never liked things to be hidden from them. The only good knowledge is that which they, not other people, possess.' He smiled sweetly at me.

'And were you guilty?'

'Oh, good heavens, yes! Of course. I had many contacts with foreigners, some of them in the highest positions. And my explorations into the world of the occult were well advanced, even then. It is why I am here now.'

'I beg your pardon?'

'I am over one hundred and forty years old. And, as you see, still in remarkably good health. I only wish that I had finished my studies earlier; then I might have presented myself as a younger man. But still, all creatures prefer some sort of life to none at all. Nobody wishes to die. Do you?'

A strange remark, half statement, half query. 'Why do you ask that?'

'Because you will. But you are still too young to realise it. One day, you will wake up and you will know. Then the rest of your life will be merely preparing for that moment. And you will spend your time trying to rectify your mistakes. The mistakes you are making now.'

'What mistakes are those?'

He smiled elliptically. 'The mistakes that will kill you, of course. I do not need to tell you what those are. You know them well enough yourself.'

'I'm quite sure I do not.'

He shrugged, uninterested.

'Why do you follow Mr Cort?'

'Who is Mr Cort?' he asked, puzzled.

'You know very well, I think. The young English architect. The palazzo.'

'Oh, him. I do not follow him. He summons me. And is a very great nuisance, I must say. I do have better things to do than dance attendance on him.'

'That is ridiculous,' I said, a little angrily. 'Of course he doesn't summon you.'

'But he does,' Casanova replied calmly. 'He really does. He is a man with many conflicts. He wishes to know about this city, and impose himself on it. He wishes to be here and away. He loves a woman who is cruel and heartless, and who dreams of his ruin. All these things call me to him, as they called his mother to me when she lay on her deathbed. I know about love and cruelty, you see, in all their forms. And I am Venice. He wants to know me. And his desire summons me to him.'

I could scarcely restrain myself from reacting to this nonsense, which he spoke so calmly. Casanova – you see, I call him that – sighed a little.

'I know about women, you know,' he said. 'Their natures. I can peer into their souls, see what lies beneath the professions of love, the lies, the demure sweetness. No other man in history has studied them as I have. I can see her thoughts. She thinks of hunting or being hunted. There is no kindness in her, and she sees only herself, never others.'

'Be quiet,' I ordered. 'I order you to keep silent. You are mad.'

'It is of no moment to me whether you believe me or not, you know,' he said. 'You will find out for yourself soon enough. I did not ask you to come here. My explorations into the occult caused me to drink in the soul of Venice, to become the city. Her spirit has extended my life. As long as Venice exists, so shall I, wandering her streets, remembering her glory. We will die together, she and I. And I see everything that happens here, even in small rooms rented for a month, or in a copse on the Lido.'

'You are not wandering the streets now,' I said with some savage satisfaction, deeply disturbed and shocked by what he was saying.

'No. For the time being I rest. And why should I wish to escape?' He smiled, and looked around him with amusement flickering on his face. 'The good doctor, it seems, is fully wedded to the best notions of gentleness for his poor inmates. I am fed well, and have to do little in return for my board and lodging, save allow myself to be measured and photographed, and to answer questions about my life. Which I have not yet decided how to do.'

'What does that mean?'

'They are most interesting questions,' he continued by way of explanation. 'They are trying to discover contradictions, impossibilities in what I say. It is excellent fun, for they go off to read my memoirs, then come back to quiz me about them. But I wrote them! Of course I know the answers better than they do. Every truth and every little fib I put in. The question is, do I tell the truth, or do I give them what they want? They so greatly desire to prove I am insane, and not who I am, that I am dreadfully sorry to disappoint them. Perhaps I should drop a few hints and contradictions into my conversation so they can conclude I am someone else entirely? It would make them so happy and grateful, and I have always desired to please. What do you think?'

'I think you should tell the truth at all times.'

'Pish, sir, you are a bore. I suppose you say your prayers every night, and ask God to make you virtuous. And you are a hypocrite. You lie all the time, except you do not even realise you are doing it. Goodness! This is a dull time to be living.'

He leaned forward, so that his face was close to mine.

'What are you in your dreams, when no one is there? What do you do in this city, which you have persuaded yourself is just a dream? How many people are you lying to now?'

I glared at him, and he chuckled. 'You forget, my friend, that I am in your dreams as well.'

'I don't know what you mean,' I said stiffly. I found that I could not answer him properly.

'Standing by a window? You don't understand it. Why didn't you turn and ask me? I could have answered, you know. I was there, you know I was. I could have told you everything.'

'How do you know about that?'