Warning: Inconsequential spoilers later on.
Dr. Conan Doyle's education as a student of medicine taught him how to observe, and his practice, both as a general practitioner and a specialist, has been a splendid training for a man such as he is, gifted with eyes, memory, and imagination. Eyes and ears which can see and hear, memory to record at once and to recall at pleasure the impressions of the senses, and an imagination capable of weaving a theory or piecing together a broken chain or unravelling a tangled clue, such are implements of his trade to a successful diagnostician. If in addition the doctor is also a born story-teller, then it is a mere matter of choice whether he writes detective stories or keeps his strength for a great historical romance as is the "White Company." Syme, one of the greatest teachers of surgical diagnosis that ever lived, had a favourite illustration which, as a tradition of his school, has made a mark on Dr. Conan Doyle's method, "Try to learn the features of a disease or injury as precisely as you know the features, the gait, the tricks of manner of your most intimate friend." Him, even in a crowd, you can recognise at once; it may be a crowd of men dressed alike, and each having his complement of eyes, nose, hair, and limbs; in every essential they resemble each other, only in trifles do they differ; and yet, by knowing these trifles well, you make your diagnosis or recognition with ease. So it is with disease of mind or body or morals. Racial peculiarities, hereditary tricks of manner, accent, occupation or the want of it, education, environment of all kinds, by their little trivial impressions gradually mould or carve the individual, and leave finger marks or chisel scores which the expert can recognise. The great broad characteristics which at a glance can be recognised as indicative of heart disease or consumption, chronic drunkenness or long-continued loss of blood, are the common property of the veriest tyro in medicine, while to masters of their art there are myriads of signs eloquent and instructive, but which need the educated eye to detect. A fair-sized and valuable book has lately been written on the one symptom, the pulse; to any one but a trained physician it seems as much an absurdity as is Sherlock Holmes' immortal treatise on the one hundred and fourteen varieties of tobacco ash. The greatest stride that has been made of late years in preventative and diagnostic medicine consists in the recognition and differentiation by bacteriological research of those minute organisms which disseminate cholera and fever, tubercle and anthrax. The importance of the infinitely little is incalculable. Poison a well at Mecca with the cholera bacillus, and the holy water which the pilgrims carry off in their bottles will infect a continent, and the rags of the victims of the plague will terrify every seaport in Christendom.
Trained as he has been to notice and appreciate minute detail, Dr. Doyle saw how he could interest his intelligent readers by taking them into his confidence, and showing his mode of working. He created a shred, quick-sighted, inquisitive man, half doctor, half virtuoso, with plenty of spare time, a retentive memory, and perhaps with the best gift of all--the power of unloading the mind of all the burden of trying to remember unnecessary details. Holmes tells Watson: "A man should keep his little brain-attic stocked with all the furniture that he has likely to use, as the rest he can put away in the lumber-room of his library, where he can get it if he wants it." But to him the petty results of environment, the sign-manuals of labour, the stains of trade, the incidents of travel, have living interest, as they tend to satisfy an insatiable, almost inhuman, because an impersonal curiosity. He puts the man in the position of an amateur, and therefore irresponsible, detective, who is consulted in all sorts of cases, and then he lets us see how he works. He makes him explain to the good Watson the trivial, or apparently trivial, links in his chain of evidence. These are at once so obvious, when explained, and so easy, once you know them, that the ingenuous reader at once fee,s and says to himself, I also could do this; life is not so dull after all; I will keep my eyes open, and find out things. The gold watch, with its scratched keyhole and pawnbrokers' marks, told such an easy tale about Watson's brother. The dusty old billy-cock hat revealed that its master had taken to drinking some years ago, and had got his hair cut yesterday. The tiny thorn-prick and fearsome footmark of the thing that was neither a child nor a monkey enabled Holmes to identify and capture the Andaman Islander. Yet, after all, you say, there is nothing wonderful; we could all do the same.
The experienced physician and the trained surgeon every day, in their examinations of the humblest patient, have to go through a similar process of reasoning, quick or slow according to the personal equations of each, almost automatic in the experienced man, laboured and often erratic in the tyro, yet requiring just the same simple requisites, senses to notice facts, and education and intelligence to apply them. Mere acuteness of the senses is not enough. Your Indian tracker will tell you that the footprint on the leaves was not a redskin's, but a paleface's, because it is marked a shoe-print, but it needs an expert in shoe-leather to tell where that shoe was made. A sharp-eyed detective may notice the thumb-mark of a grimy or bloody hand on the velvet or the mirror, but it needs all the scientific knowledge of a Galton to render the ridges and furrows of the stain visible and permanent, and then to identify by their sign-manual the suspected thief or murderer. Sherlock Holmes has acute senses, and the special education and information that makes these valuable; and he can afford to let us into the secrets of his method. But in addition to the creation of his hero, Dr. Conan Doyle in this remarkable series of stories has proved himself a born story-teller. He has had the wit to devise excellent plots, interesting complications; he tells them in honest Saxon-English(note) with directness and pith; and, above all his other merits, his stories are absolutely free from padding. He knows how delicious brevity is, how everything tends to be too long, and he has given us stories that we can read at a sitting between dinner and coffee, and we have not a chance to forget the beginning before we reach the end. The ordinary detective story, from Gaboriau or Boisgobey down to the latest shocker, really needs and effort of memory quite misplaced to keep the circumstances of the crimes and all the wrong scents of the various meddlers before the wearied reader. Dr. Doyle never gives you a chance to forget an incident or miss a point.
-- Dr. Joseph Bell