I have stood at the top of the Primorsky Stairs in the great port of Odessa, and at the bottom. It is a most peculiar structure, fashioned so that as one looks down, its two hundred steps disappear beneath the weight of foreshortening – for the staircase actually grows wider as it goes down to the sea, and one sees only broad, stone landings from above. Yet, looking up, it seems a ladder to the heavens, vanishing into the distance as the sides move closer together.

I pride myself that I am not deceived by appearances, by tricks and the designs of men. Disguise does not fool me, though I have penetrated the secrets of many by adopting it myself. The Trepoff sisters little suspected that the French chemist, who supplied them with the poison they meant to administer to a second uncle as they did to the first, was an English detective. The jealous Parsee who murdered both Atkinson twins because he could not tell them apart, might have escaped discovery had he understood that his wife knew perfectly well which was which. The lack of male heirs born to the house of Orange-Nassau over the past thirty-seven years is a mere statistical anomaly, a quirk of nature that cannot be overturned by its princesses resorting to witchcraft.

I see, and observe, only the facts.

Watson would never have got by in Odessa: he has no Russian at all and his French stumbles about a conversation like a drunkard in the road. Were I to have been seeking the pen of my aunt or the location of the post office, he might have done well enough, but otherwise... He would have been laid low with a bout of enteric fever in Trincomalee and found Holland, well, flat as a pancake.

Better that he resides behind his own four walls, now, and I behind mine. It was a comfortable enough arrangement for a while: sharing quarters and adventures, tea and toast, murder and laughter. Nevertheless, he has needs I could never meet - a longing for sanity; a small, soft hand to brush his hat; to be master in his own house.

By contrast, I have no needs I cannot supply, whether at the end of a needle or the approach of a client. What use is sanity to me – another word for lack of imagination, for refusing to look into the abyss and allowing it to look back until you understand the threads that hold the human mind together and can unpick them one by one? What use would a wife be to me, or I to her? I am cold, perhaps, but not so deliberately unkind. As for being master, I can do that just as well in somebody else's house.

Yes, we have both of us chosen wisely - observed the facts of our disparate natures and acted accordingly.

Still, I have dreamed, in the grey half-sleep after my morning dose wears off, that I stood at the foot of that great staircase where two walls, separated by the needs of men to make, each one, his own journey in this world, grow closer – or at the top, where journey's end seems not so far.

So, too, towards evening when men wend their way homeward, I have sometimes thought that I heard a familiar tread on another, far humbler staircase.