The towering exception, of course, is Professor Moriarty. It may be that I am almost alone in having no time for Moriarty, and regard him as little more than a desperate device by an author exceptionally keen to kill off his hero and bring his enslavement to an end. Conan Doyle’s haste, I feel, made him careless in his depiction of the character and Moriarty as a result is a largely unsatisfying invention. Presumably Conan Doyle wanted to kill Holmes off by having this final showdown with his equal but opposite, in a Manichaean struggle between good and evil, dark and light, in which each cancelled out the other. The trouble is that Moriarty is so unconvincing that the contrast does not work. The idea of the master criminal, the ‘Napoleon of crime’, to use Conan Doyle’s memorable phrase, has been returned to again and again and never with any success except in espionage stories which are structurally capable of sustaining such figures. In inventing Moriarty, Conan Doyle abandoned almost all of the characteristics that made the earlier stories so successful. Holmes works because he is an outsider who controls nothing, and because the crimes he investigates have an individual, realistic tinge to them that makes them believable and enthralling. In ‘The Final Problem’ there is no particular crime or event at the centre: we are told of Moriarty’s great evil and intelligence, but are never shown these qualities in operation. With Moriarty, Conan Doyle steps from detective story into thriller, drama into melodrama. In earlier tales there are references to Holmes’s great cases, saving Scandinavian kings or confounding international swindlers, but Conan Doyle had too much sense to turn them into stories in their own right: for these he concentrates on the small-scale, almost the domestic incident. Holmes’s methods are simply not suited for grand conspiracy, and it is notable that in ‘The Final Problem’ there is no detection at all.

For in the stories detection is essential, even if it is not always central to the resolution of the tale. What Conan Doyle created was the perfect positivist, the embodiment of Victorian faith in rationality and science, convinced that the right combination of method and reason could overcome all obstacles. Even though our own trust in science is not what it was, and our faith in human rationality has taken a battering since the middle of the twentieth century, Sherlock Holmes still presents an attractive enough figure, although now viewed with a sentimental affection rather than seen as an almost aggressive blueprint for the future. For the methodology which Holmes employs was then radical and novel, even though it has now infiltrated all areas of life to such an extent that it is scarcely even noticed. The essence of the stories is the willingness to ignore the big picture and, instead, to apply reason to an accumulation of apparently insignificant details which then reveal the truth when properly analysed. Indeed, the more insignificant the detail, the more pure and useful it is, hence the importance of dogs that don’t bark in the night, footprints, shiny cuffs, and so on.

The significance of this is that Conan Doyle moved into the popular realm a method of thought which was of incalculable importance in several areas of intellectual life. The Holmesian method, after all, had already been put into practice in real criminal work by criminal anthropologists who began by measuring heads, ears and so on, initially with the aim of identifying criminal types for preventive incarceration, and then more modestly merely to identify individuals after crimes had been committed. The practice was later refined by Sir Francis Galton and Alphonse Bertillon, who developed fingerprinting and handwriting analysis respectively – classic cases of insignificant details being made to yield significance – for the same purpose. Fingerprinting, which received little publicity until Galton’s book on the subject in 1892, is never referred to in these stories, but Holmes is an early analyst of the particularities of the typewriter, which, he says in ‘A Case of Identity’, ‘really has quite as much individuality as a man’s handwriting’.

On the other hand, hereditarian arguments – suggesting that criminals are born and not made and can be identified by the study of physical characteristics and family trees – are of scant importance and, in the stories presented here, are referred to only in the case of Moriarty. This is perhaps a surprising omission, as eugenics, the science of race and its application, was then at the very cutting edge of research – Galton, for example, spent much of his career writing on methods of defending and preserving the race, and scientists throughout Europe confidently asserted the need for radical measures to prevent racial degeneration. Had Holmes identified criminals by using his powers of observation to spot degenerative tendencies to crime he would have been acting fully in accordance with the advanced scientific reasoning which he employs in other areas.

Yet Conan Doyle refrains from any such device; even in The Hound of the Baskervilles, where atavism and degeneration are important motifs, they play no significant part in solving the mystery – identifying the villain through his resemblance to an old family portrait is quite an old trick of plotting which owes little to fin-de-siècle scientific thought. Equally importantly, Conan Doyle declines the frequent opportunities to make orthodox remarks about the characteristics and inferiority of other races – indeed, the implications in a story such as ‘The Yellow Face’ are quite the opposite of orthodoxy. It is a quiet expression of a natural humanity which stands very much in his favour.

Instead of such notions, it is hard external evidence of a particular kind which is of importance: ‘Never trust to general impressions, my boy, but concentrate yourself upon details,’ he tells Watson in ‘A Case of Identity’, an aphorism which sums up not only the whole of the Holmes stories but also much of late nineteenth-century intellectual life, from forensic science through German philological studies of the Bible and on to the dating of ancient history through potsherds or the uncovering of the fossil record through bone fragments. Holmes himself draws the parallel in ‘The Five Orange Pips’ when he remarks that ‘As Cuvier could correctly describe a whole animal by the contemplation of a single bone, so the observer who has thoroughly understood one link in a series of incidents, should be able accurately to state all the other ones, both before and after.’

As the Italian scholar Carlo Ginzburg has noted, perhaps the most precise parallel with the Holmesian method comes with Sigmund Freud and the development of psychoanalysis. The parallel even extends to the literary form, for while Holmes is at his best in the short story, so Freud reaches his height through lectures and above all the celebrated case studies which, in narrative drive and economy of expression (and perhaps, in imaginative invention), rank as literary masterpieces in their own right. The similarities are indeed extraordinary: detective and analyst are required to set aside their own characters and become almost disembodied intellects to pursue the truth, preserving a distance from their clients; they both maintain consulting-rooms to which clients with troubles come in search of relief; while Freud insists on medical confidentiality, Holmes maintains that ‘I extend to the affairs of my other clients the same secrecy which I promise to you in yours’ (‘The Noble Bachelor’). Moreover, both all but break that assurance in order to publicize their method, Freud by writing up his cases, Holmes by allowing Watson to do so.

Above all, both see the significance of the insignificant, and use it to pioneer a new method of investigation which has since had many imitators but few superiors. For while Holmes pursues ‘the little things’ in pursuit of the truth, so Freud considers previously overlooked trifles – such as facial tics, jokes and dreams – to delve into the unconscious and reconstruct a past which is otherwise completely obscured. The classic Freudian case study begins with a neurosis and works back through details to its point of origin; the best Holmes stories begin with a crime (‘Silver Blaze’, ‘The Naval Treaty’), or a piece of inexplicable behaviour (‘The Red-Headed League’), or a disappearing fiancé or wife (‘A Case of Identity’, ‘The Noble Bachelor’), or a mysterious death (‘The Five Orange Pips’, ‘The Speckled Band’), and again works back to the source of the disturbance.