The servant who had been solicitous of Kant's safety that morning on the river bank was peeping from the rear corner of the coach. His large, strong face, which had appeared so expressionless then, now seemed tense and drawn.

'Johannes Odum, isn't it?'

With a pointed glance he indicated that I should join him behind the coach.

'Your master will not condone time-wasting,' I warned him.

'A word, sir, no more,' he insisted. He gestured towards the vehicle and its pa.s.senger with his thumb. 'Recent events have been a trial for him, sir. What we saw this morning down by the river is good for no man, sir, least of all a gentleman of his age and delicate nervous disposition.'

'You were there, you saw him yourself,' I whispered. 'Professor Kant may be frail, but he seems to be holding up.'

Perhaps I ought to have warned the servant that the danger was past, and that the case was closed, but I had no intention of wasting such momentous news on Kant's valet before I had told it to the man himself.

'He's been working night and day at this investigation, sir,' the servant replied. 'All night sometimes...'

'All night?' I interrupted. 'Doing what?'

'Writing, I believe, sir.'

I thought of the treatise that Herr Jachmann had mentioned, his incredulity regarding its existence. 'Do you know what he is writing?'

Johannes Odum shrugged his broad shoulders dismissively.

'He's in danger, sir,' he insisted. 'Real danger. He was seen with you this morning by the river. Now, you'll be seen travelling in his coach. And before we left the house this morning, I found something that you should see...'

'Johannes!' The fretful shout made both of us start. 'Where is Procurator Stiffeniis?'

I signalled to the servant to run around the other side of the vehicle, while I stepped out and attracted Professor Kant's attention.

'Here I am, sir,' I said brightly. 'I left some papers behind in the office, and had to go back for them. Do you mind if Sergeant Koch joins us?'

I nodded to Koch to stand forward.

'Of course not,' Kant replied with impatience. 'We must hurry. The way is long, and it is extremely cold.'

'Off to Siberia, are we, sir?' I joked. I knew I could not fail to hit the mark. Kant's hunger for information and gossip was as renowned as his clock-watching. The news of the ship that was about to moor in Pillau port could not have escaped him. It had featured prominently in all the recent Prussian papers.

'Not so far,' he replied with a smile, 'but just as cold.'

I laughed heartily. I was in an excellent humour. The case was over, except for the red tape. Even if they managed to escape the hangman, Ulrich and Gerta Totz would be deported to the frozen wastes. I had no idea where Professor Kant was taking us, nor what he intended to show us. Whatever it might be, I thought, humouring the old gentleman would add nothing material to the investigation. But nor would it detract.

As the coach sped forward, I expected him to ask me about the progress I had made that afternoon. Was he not curious to know what had happened? He had been so openly sceptical that morning, yet he had urged me to interrogate Ulrich and Gerta Totz. Surely he would wish to know what they had said?

'Do you like your new lodgings?' he asked suddenly. 'They hardly compare with the delights of The Baltic Whaler, I'd wager. Frau Totz is renowned for her roast pork.'

Was he teasing me? Was my ex-landlady's cooking all that interested him?

'The inn was certainly comfortable,' I conceded uncertainly.

'I knew you'd feel at home there,' Kant said with a warm smile. 'Of course, the Fortress of Konigsberg is another matter altogether.'

Was this the behaviour that had so disturbed Herr Jachmann? Kant seemed to be absorbed in details which were of no importance, concerned about matters of which he could have no personal knowledge. He had dined in his own home and nowhere else for the last twenty years, as all the newspaper sketches of his doings habitually reported. Celebrity has no secrets from the intrusions of the press.

'What a depressing building the Fortress is!' he said next, his mood shifting suddenly. 'The sight of it used to put the wind up me when I was just a little child. Mother and I were obliged to pa.s.s each morning on our way to the Pietist Temple. The fear I felt, she said, was naught compared to the fear I'd feel the day I had to stand before my Maker and look into His eyes!'

Professor Kant looked out of the window like a lost child. The Fortress had fallen far behind, but it might still have been visible before his eyes. 'Will you be able to sleep in there tonight, Hanno? They say that the place is haunted by the victims of the Teutonic Knights who died in the dungeons.'

What could I reply? Koch and I exchanged glances, but neither dared say a word. As our coach clattered and rattled over an ancient wooden bridge, dense fog swirled in twirling clouds above the still waters of a dark moat. Only the towering keep of the Fortress on the hill above was visible in the fading afternoon light. The battlements seemed to peep over a solid wall of low clouds.

Kant glanced in my direction. 'Nearly there!' he exclaimed gaily as the coach turned sharply right and crossed another bridge. Clearly, he was excited by the prospect of what lay before us. 'I suppose that you have been counting the bridges?' he said.

'Bridges, sir?' I had no idea what he was talking about.

'Surely you know the problem?' he replied. 'Before he died, the great mathematician, Leonhard Euler, questioned whether it were possible to trace a route through Konigsberg which crossed the nine bridges spanning the River Pregel without ever using the same bridge more than once. You ought to try it while you are here.'

I began to remind him of the reason which had brought me to Konigsberg, but he had no mind for me. 'When I began to teach at the University,' he went on, 'I won a bet with a colleague who had been a great friend of the mathematician. He told me that, in fact, Euler himself didn't know the answer! Well, I provided two solutions to the problem...'

He did not finish. Turning to me instead, he laid his hand on my arm, and asked urgently: 'What have you to tell me about the Totzes?'

For some moments, I knew not how to reply. Should I tell him that the case was solved, the guilty closed in their cells, awaiting judgement? That the Devil's claw, whatever it might prove to be, was irrelevant?

'The husband confessed, sir. And quickly too,' I replied. With care, limiting myself to the sequence of events, suppressing the cry of victory which hovered on my lips, I related the facts to Kant as neutrally as possible.

'So, that's that,' he said. 'A political plot is at the root of all the evil which has poisoned Konigsberg. Acts of terrorism aimed at...'

He halted abruptly, and looked at me.

'Aimed at what? Did the culprits reveal their final objective?'

'Not in so many words, sir,' I admitted. 'Ulrich Totz seems to believe that the fear generated by these murders will weaken the faith of the people in their rulers and provoke some sort of a revolution. I suspect that he targeted people who were known for their opposition to the French.'

Professor Kant sat back in his seat. He beamed with delight.

'Oh, I see! How clever of him! And he described the weapon that he used to kill Morik, I suppose?'

I shifted uneasily on the leather bench.

'A hammer, sir.'

My answer seemed to amuse Kant even more. 'A large hammer, Stiffeniis, or a small one?' he asked.

'It...it was only a preliminary interrogation,' I stammered. I had thought to win his praise. Instead, his penetrating mind had laid bare the limits of my method of proceeding. 'Totz did admit that he had used various weapons to kill the other victims.'

'Not one alone?' Kant frowned.

'Whatever came to hand, he said,' I added quickly. 'Of course, sir, I will question him until all the details emerge.'

'Details are of the utmost importance,' Kant confided. 'The King will want to know the exact strength and number of his enemies.'

Was he being sarcastic? I felt like a student who has just been handed back an essay by a tutor and told that the work was good, very good, though it should have been a great deal better. Suddenly, Kant laughed out loud. He did not say what amused him. This mercurial humour of his was new to me, and I was not rea.s.sured by it. Nor was Koch, as I could see quite clearly by the expression on his face.

'I am glad that you have found the high road to Truth,' said Kant. 'Did you, by any chance, ask Ulrich Totz about the Devil's claw, as the people call it?'

'Herr Procurator cannot be expected to complete his investigations in a single day, sir,' Sergeant Koch interrupted. His deference to my authority was as p.r.o.nounced as his loyalty to Procurator Rhunken had been. The bureaucracy here in Prussia is famed for producing such men. They are obedient and subservient to a fault. And sometimes they are blunt, too.

'Whatever this Devil's claw may be,' Koch went on, 'whatever the common people say about it, it hardly seems relevant, Professor Kant. Procurator Stiffeniis has unmasked the plot.'

'Dear Sergeant Koch,' Kant returned mildly, 'do not presume too far. In my experience, there is more truth in the common voice than anywhere else on Earth.'

'Ulrich Totz admitted killing the boy,' Sergeant Koch replied staunchly. 'He admitted murdering the others. Herr Stiffeniis has caught his man, sir.'

To my surprise, Professor Kant showed no resentment at this reb.u.t.tal. He nodded thoughtfully. 'I can understand your reservations about the utility of what I intend to show you, Herr Koch,' he continued. 'And I appreciate your openness where my young friend shows only dutiful reticence. I am certain that Stiffeniis shares your opinion. I would ask you both to be patient for a short while longer. What you are about to see is the fruit of the most original research that I have ever undertaken in my life.'

My heart beat faster. Was Immanuel Kant going to show me what he had been hiding from his best and closest friends?

'A masterpiece, I am sure, sir,' I said with warmth. 'Any book from your pen...'

'A book?' Surprise was evident on his hollow face. 'Is that what you are expecting to see, Stiffeniis?'

'The world has waited too long for a new opus, sir,' I answered.

He made no immediate reply. When he did speak, he seemed even more animated than before. 'A book...A book! Why not?' he said, resting his chin on his bunched fist. 'And what could be the t.i.tle? Why, given the circ.u.mstances, a Critique of Criminal Reason, I think.'

'I am eager to read it,' I enthused, as the coach laboured up the hill.

Kant was smiling gleefully, his lips drawn back tightly to reveal the few yellow, pointed teeth he still possessed. I must confess, it was not a pleasant sight.

'You have taken possession of Rhunken's office, I imagine. Have you read the reports that he cobbled together regarding these murders?'

'I did so yesterday,' I began eagerly. 'They have been most useful, sir. Indeed his theory seems to be confirmed by what Totz, the innkeeper, confessed this afternoon...'

'A political plot? Is that what you think lies behind these deaths?' Kant interrupted me with a dismissive wave of his hand. 'Vigilantius came closer to the truth!' He uttered these words with an energy which was very close to rage. 'You lost patience with him last night. You should have stayed until the end. Herr Rhunken is a magistrate of the old school. He is an information-gatherer, nothing more. He hopes to frighten the truth from people, and sometimes he succeeds. But not in this instance. His dull imagination is no match for the killer's. Vigilantius has discovered a great deal more, but you refused to take his insights into consideration.'

I glanced at Koch. His face was taut, the muscles clenched and stiff. Clearly it cost him a great deal not to speak out in defence of the dead man he had served so faithfully and for so long. But one thing was certain: Professor Kant had not been informed of the magistrate's death.

'Well?' Kant niggled. 'Why didn't you stay?'

'I considered it a pantomime, sir,' I protested uncertainly.

'Pantomimes sometimes represent a truth,' he replied. 'I expected you to be put off at the beginning, but I hoped that you would learn something useful from Vigilantius. I sent Rhunken's official reports to you for the same purpose.'

'I beg your pardon, sir?'

I could find no thread of coherence in his arguments. What could be the connection between Vigilantius and the police reports that I had been allowed to read on the journey to Konigsberg?

He leaned close and spoke quietly. 'I knew that I could rely on your sense of duty. Who can refuse a commission from the King? Especially a magistrate who has chosen to hide himself away in a tiny village near the borders of West Prussia. What is the name of the place? Lotingen?'

For a moment I feared that he might be about to ask me why I had chosen never to return to Konigsberg after our first meeting seven years before, why I had never made the effort to write to him. The possibility that he might know about Jachmann's interference in our affairs threw me into a panic. What could I tell him? In frenzied haste, I searched for excuses, ranging from my own bad health, to G.o.d forgive me! serious diseases that Helena or the children might have suffered.

But he did not bother to ask. He had other matters on his mind.

'I tried to whet your appet.i.te for the obscure side of human behaviour with the help of those reports, Hanno,' he went on. 'I hoped that you would be intrigued by the strangeness of these deaths. I remember well at our first meeting, you showed a natural inclination for...how shall I call it, mystery?' He leaned back in his seat. 'I hoped that you would be puzzled. Not so much by what you'd read, but by what you did not read in those reports.' He started to number off the items on his fingers.

'Why was there no explanation of how the victims had died? Why was no mention made of the weapon used? Why was no hypothesis offered which might suggest a common motive behind all those murders? There was no question of theft or of pa.s.sion, no apparent connection linking one victim to another. You cannot fail to have realised that what was happening in Konigsberg had something peculiar about it. A magistrate of Herr Rhunken's fame was unable to resolve the enigma. Oh, I'd not deny that Rhunken did his job. He did what he was capable of doing. But his heavy feet never left the ground. His plodding intelligence was no match for the killer's. Sometimes such magistrates are fortunate, I have no doubt, but not in this instance.' He looked at me inquisitively.

'If you want to understand what is going on here, my young friend, you must learn to soar. You must pay attention, even to the most obscure and mysterious of sources available to you. Even to the likes of Vigilantius. If you continue to dig for reasons, to look for explanations, to chase proofs as you began to do the minute you arrived in Konigsberg you'll get no closer to the truth than your predecessor did.'

His voice gradually faded away as he spoke. He was disappointed, I could tell. I had failed him in some way, though I was unable to say exactly how I had fallen short of his expectations. But suddenly, he shifted in his seat and changed his course, like a fish you think you have safely caught in your bare hands that darts away in another direction, leaving you with just an empty swirl in the water.

'By the way, how is your father?' he asked.

It was dark inside the carriage, and I was glad of that. I felt the blood drain instantly from my face. It was the second time that he had referred to the tragedy in my family. What strange a.s.sociation of ideas brought the question to his lips just then? And why was he so lacking in curiosity about the new family I had made? Were my wife and my children of no interest to him? I had baptised my only son in his honour. It was as if that part of my life did not exist. Instead, he harked back continually to the old life, the old me, the Hanno Stiffeniis that he had helped to exorcise seven years before.

'I have heard, sir, that he is somewhat better,' I replied, though Kant did not appear to be listening. He seemed to be following a delicate pattern that his mind had already traced out for him. He waved his forefinger in the air, following the movement with his eyes, as though his mental and his physical states were wholly separated, and equally fascinating, the one to the other.

Just then, fortunately, the coach began to slow down.

'At last! We have arrived!' he exclaimed, interrupting his private reflections with sudden animation. 'Let's waste no more time.'

Johannes pulled down the folding steps, and helped his master into the dark lane. I had no idea where we were, but then I caught a glimpse of the towering bulk of the Fortress again. We seemed to have circled around the defensive walls and come at the Fortress from another direction, standing beside a miserable hovel that flanked an approach road to the portcullis. The building might have been used centuries before as a customs post. It was in a remarkable state of dereliction and ought to have been pulled down. I was baffled, wondering what had driven Herr Professor Kant to choose such a forlorn place to work upon his final masterpiece?

With an eager nod from his master, Johannes Odum produced a large key from his pocket, and proceeded with some difficulty to unlock the ancient, worm-eaten door.

'Wait here,' Kant said to his man. 'Now, Stiffeniis, if you lend me your arm, perhaps Sergeant Koch could step inside and strike a light? There's a lantern hanging just inside the door.'

The cobbled road was slippery underfoot with packed ice and snow and a thin top-sprinkling of frost, as treacherous as a road could be. If I needed proof of Immanuel Kant's age and physical frailty, I had it then. He rested his feather-like weight on my arm as we stepped inside the door, where Koch was waiting with a storm-lantern raised high above his head.

'Do not touch anything,' Kant warned.

We were in an abandoned store-room of some sort. Broken armament boxes had been dumped in a large, careless heap on one side of the room. Cobwebs hung like shimmering shrouds from the ceiling. Dust lay in a thick blanket over everything else. In the centre of the room, a large rat caught by the neck in a trap had been stripped to a skeleton by its luckier fellows.

'Go ahead, Koch,' Kant ordered. 'We will follow you.'

He pointed towards a narrow, vaulted tunnel, whose once-white walls were stained and black with mould and smoke. Dropping my arm as if possessed of a demonic energy, he set off at a shuffling trot behind Sergeant Koch, and I was obliged to accelerate to keep pace with him. Our pa.s.sage through this tunnel was made more difficult by the lowness of the brick vault, which grazed the top of Kant's three-cornered hat, whilst Koch and I were obliged to stoop. My nose tingled with the pungent odour of rot, and there was a sharp, acidic smell underlying it. If Doctor Faustus and his familiar, Mephistopheles, had leapt out to welcome us in the gloomy depths of the place, I would not have been in the least surprised.

As we entered a large room at the end of the corridor, and Koch held up his light, I caught my first glimpse of the voluminous alembic jars and the serpentine gla.s.s-tubes which gleamed upon the shelves, together with a neat pile of boxes set in an orderly stack on a workbench.

'Feel the cold,' Kant enthused. 'Siberia is closer than you think!'

He ordered Koch to take a spill from his lantern and light the lamps which were hung at intervals around the walls. As the sergeant added light to light, the objects a.s.sembled in that room began to stand out more clearly from the gloom.

Professor Kant turned towards the far wall.

'Now, Stiffeniis, let me introduce you to those who are obliged to dwell down here in this unwholesome twilight world,' he said.

From the darkest corner of the room, vacant, watery eyes stared fixedly back at us in the pale, flickering glow of the lamplight.