Logic and Deduction

‘THE SIGN OF FOUR’
Holmes’s remarkable faculties are shown at their best time and again when he is making deductions from evidence that seems to the rest of us to
yield little of value. Indeed, so accurate are the conclusions he draws that at times it seems almost as if he has mystical powers or psychic abilities.
But what is the process of logical deduction?
Accumulate evidence.



By using his finely-honed powers of observation, Holmes was able to gather vast amounts of information from even the most unpromising of
sources.
Ask the right questions.
Holmes formulated clear questions in his head that he wished to answer. For instance, what does this person’s clothing tell me about where
they come from or what sort of job they have? What does a dog’s silence signify? Why might a red-headed pawnbroker be required to copy
out an encyclopaedia for several hours a day?
Formulate hypotheses.
Consider an otherwise well-dressed doctor who arrives at Baker Street carrying a rugged walking cane and wearing shoes covered in
compacted mud of a colour not usual in the capital. Why might this be? Does he not look after his shoes properly? Are London’s shoeshine
boys on strike? Has he come in a hurry from an appointment in the country?
Evaluate hypotheses.
The doctor is smartly dressed so it is unlikely that he simply doesn’t pay attention to his shoes. You went out for a walk earlier and saw a
shoeshine boy so you know they are not on strike. The doctor does seem flustered, however, as if he has rushed to Baker Street.
Reach a conclusion.
Ask the doctor what has caused him to hurry away from his rural practice.
On the occasion of Watson’s very first meeting with Holmes in A Study in Scarlet, the Great Detective gives a master class in logical deduction.
Watson had arrived in London looking for lodgings and Holmes was searching for someone with whom to share rooms, so the two men were
introduced by a mutual friend named Stamford:
‘Dr. Watson, Mr. Sherlock Holmes,’ said Stamford, introducing us.
‘How are you?’ he said cordially, gripping my hand with a strength for which I should hardly have given him credit.
‘You have been in Afghanistan, I perceive.’
‘How on earth did you know that?’ I asked in astonishment.
‘Never mind,’ said he, chuckling to himself. ‘The question now is about haemoglobin. No doubt you see the significance of this discovery of
mine?’
And so the story progresses. The two men agree to take rooms together and Holmes eventually divulges to his new companion just how he
achieved such a remarkable insight:
I knew you came from Afghanistan. From long habit the train of thoughts ran so swiftly through my mind, that I arrived at the conclusion without
being conscious of intermediate steps. There were such steps, however. The train of reasoning ran, ‘Here is a gentleman of a medical type,
but with the air of a military man. Clearly an army doctor, then. He has just come from the tropics, for his face is dark, and that is not the natural
tint of his skin, for his wrists are fair. He has undergone hardship and sickness, as his haggard face says clearly. His left arm has been injured.
He holds it in a stiff and unnatural manner. Where in the tropics could an English army doctor have seen much hardship and got his arm
wounded? Clearly in Afghanistan.’ The whole train of thought did not occupy a second. I then remarked that you came from Afghanistan, and
you were astonished.
Conan Doyle took his inspiration for such deductive brilliance from a real-life source: Dr Joseph Bell. Conan Doyle studied medicine at Edinburgh
University under Bell in the 1870s and would later write to tell him, ‘It is certainly to you that I owe Sherlock Holmes… I do not think that his analytical
work is in the least an exaggeration of some effects which I have seen you produce in the out-patient ward.’
Bell’s great trick was to diagnose a patient and discern his background without taking any form of history. It was said he could spot a sailor by his
rolling gait, a traveller’s route by the tattoos he bore, and any number of occupations from a glimpse at a subject’s hands. As if to prove the point,
Conan Doyle once saw him correctly place a patient as a non-commissioned officer, recently discharged from the Highland Regiment posted in
Barbados.
In A Study in Scarlet, Watson arose one morning to find a magazine on the table of 221B Baker Street. His eye was drawn to one particular article
bearing a pencil mark at the heading:
Its somewhat ambitious title was ‘The Book of Life’, and it attempted to show how much an observant man might learn by an accurate and
systematic examination of all that came in his way. It struck me as being a remarkable mixture of shrewdness and of absurdity. The reasoning
was close and intense, but the deductions appeared to me to be far fetched and exaggerated. The writer claimed by a momentary expression,
a twitch of a muscle or a glance of an eye, to fathom a man’s inmost thoughts. Deceit, according to him, was an impossibility in the case of one
trained to observation and analysis. His conclusions were as infallible as so many propositions of Euclid. So startling would his results appear
to the uninitiated that until they learned the processes by which he had arrived at them they might well consider him as a necromancer.
Watson was initially rather dismissive of its contents, describing it as ‘ineffable twaddle’ and claiming not to have ‘read such rubbish in my life’. A
moment or two later, though, Holmes revealed that he himself was the author of the piece. As such, it is invaluable to students of the Great
Detective for its explanation of his deductive process:
‘From a drop of water,’ said the writer, ‘a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or
the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it. Like all other arts, the Science of
Deduction and Analysis is one which can only be acquired by long and patient study, nor is life long enough to allow any mortal to attain the
highest possible perfection in it. Before turning to those moral and mental aspects of the matter which present the greatest difficulties, let the
inquirer begin by mastering more elementary problems. Let him, on meeting a fellow-mortal, learn at a glance to distinguish the history of the
man, and the trade or profession to which he belongs. Puerile as such an exercise may seem, it sharpens the faculties of observation, and
teaches one where to look and what to look for. By a man’s fingernails, by his coat-sleeve, by his boots, by his trouser-knees, by the callosities
of his forefinger and thumb, by his expression, by his shirtcuffs – by each of these things a man’s calling is plainly revealed. That all united
should fail to enlighten the competent inquirer in any case is almost inconceivable.’
‘Yes; I have a turn both for observation and for deduction,’ Holmes told Watson. ‘The theories which I have expressed there, and which appear to
you to be so chimerical, are really extremely practical – so practical that I depend upon them for my bread and cheese