Improving Your Deductive Skills

‘“When I hear you give your reasons,” I remarked, “the thing always appears to me to be so ridiculously simple that I could easily do it myself,
though at each successive instance of your reasoning I am baffled until you explain your process.”’
‘A SCANDAL IN BOHEMIA’
To improve your own deductive powers, you can do no better than read the entire Sherlock Holmes canon. School yourself in the many incredible
examples of deductive reasoning that litter the stories and aim to mirror as many of the master’s techniques as humanly possible.



 To watch the
Great Detective in action is never less than a delight, as Watson himself recalled in ‘The Adventure of the Speckled Band’:
I had no keener pleasure than in following Holmes in his professional investigations, and in admiring the rapid deductions, as swift as
intuitions, and yet always founded on a logical basis with which he unravelled the problems which were submitted to him.
There are far too many examples to cite here, but virtually anything could serve as useful evidence for Holmes. Here was a man who could discern
vast amounts about a suspect from the cigar ash he left scattered at a crime scene, who could calculate a man’s height from the implied stride
length provided by a set of footprints, and who could even (in ‘The Adventure of the Beryl Coronet’) unravel ‘a very long and complex story … written
in the snow in front of me’.
There are one or two grand set pieces worth analysing in the pursuit of learning Holmes’s secrets. The Sign of Four offers up a particularly
notable example. Watson hands Holmes a pocket watch and challenges Sherlock to provide ‘an opinion upon the character or habits of the late
owner’. Holmes begins by complaining that the watch has recently been cleaned, robbing him of his best evidence. This is, as we might suspect,
simply a bit of showmanship on his part.
‘Subject to your correction, I should judge that the watch belonged to your elder brother, who inherited it from your father,’ he begins.
‘That you gather, no doubt, from the H. W. upon the back?’ Watson fires back.
‘Quite so. The W. suggests your own name. The date of the watch is nearly fifty years back, and the initials are as old as the watch: so it was
made for the last generation. Jewellery usually descends to the eldest son, and he is most likely to have the same name as the father. Your
father has, if I remember right, been dead many years. It has, therefore, been in the hands of your eldest brother.’
So far, so logical. But then comes a leap in Holmes’s deductions that at first sight seems beyond reasonable:
He was a man of untidy habits, – very untidy and careless. He was left with good prospects, but he threw away his chances, lived for some time
in poverty with occasional short intervals of prosperity, and finally, taking to drink, he died. That is all I can gather.
Initially, Watson is knocked back on his haunches, distraught that such a painful personal episode has been so casually revealed. He accuses
Holmes of being ‘unworthy’, suggesting he had made prior enquiries into his family’s past or else had resorted to guess work. Holmes soon
corrects his companion:
What seems strange to you is only so because you do not follow my train of thought or observe the small facts upon which large inferences may
depend. For example, I began by stating that your brother was careless. When you observe the lower part of that watch-case you notice that it
is not only dinted in two places, but it is cut and marked all over from the habit of keeping other hard objects, such as coins or keys, in the
same pocket. Surely it is no great feat to assume that a man who treats a fifty-guinea watch so cavalierly must be a careless man. Neither is it
a very far-fetched inference that a man who inherits one article of such value is pretty well provided for in other respects.
It is very customary for pawnbrokers in England, when they take a watch, to scratch the number of the ticket with a pin-point upon the inside
of the case. It is more handy than a label, as there is no risk of the number being lost or transposed. There are no less than four such numbers
visible to my lens on the inside of this case. Inference – that your brother was often at low water. Secondary inference, – that he had occasional
bursts of prosperity, or he could not have redeemed the pledge. Finally, I ask you to look at the inner plate, which contains the key-hole. Look at
the thousands of scratches all round the hole, – marks where the key has slipped. What sober man’s key could have scored those grooves?
But you will never see a drunkard’s watch without them. He winds it at night, and he leaves these traces of his unsteady hand.
What you can’t learn about deduction from the Holmes stories is probably not worth knowing, but here are a few other hints and tips to help:
We all make deductions every day. If we turn on a light switch but no light comes on, we deduce that we need to put in a new light bulb or there
is a problem with the electrics. And if we get to the railway station at rush hour and see an empty platform, we might well deduce that a train
has just gone or there are no trains running. At the heart of logical deduction is an ability to make leaps of the imagination. Break the process
down into manageable stages.
The more evidence, the better you can test out a hypothesis.
The devil is in the detail. It is very often the tiniest details that give away the biggest truths.
Trust in your intuition, but only up to a point. ‘It was easier to know it than to explain why I know it,’ Holmes said on one occasion. ‘If you were
asked to prove that two and two made four, you might find some difficulty, and yet you are quite sure of the fact.’ However, it is always well to
subject your intuitions to thorough analysis to make sure that what you believe to be a fact is not merely a heartfelt conviction not borne out by
the evidence.
Don’t allow your deductive reasoning to be clouded by your personal feelings or prejudices.
Fit your theories to the facts. Do not fit the facts to your theories. In the first instance, you are reading the evidence. In the latter, you are
rewriting the evidence simply to accord with your own preconceived ideas.
Beware of the ‘conjunction fallacy’. This is when two or more events that could happen together or separately are considered more likely to
happen together than separately. A classic example was provided in a study by the psychologists Amos Tversky and Daniel Kahneman. They
provided respondents with the following description: ‘Linda is thirty-one years old, single, outspoken, and very bright. She majored in
philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear
demonstrations.’ They then asked their respondents which was the more probable scenario: 1) Linda is a bank teller, or 2) Linda is a bank
teller and is active in the feminist movement. Eighty-five per cent of respondents went for the second option but by the laws of probability, the
answer must be the first.
Just because your conclusion seems odd, even incredible, does not mean it’s wrong. As Holmes noted in ‘A Case of Identity’: ‘Life is infinitely
stranger than anything which the mind of man could invent.’ Don’t be embarrassed by the strangeness of a hypothesis if the evidence is there
to support it.
Deduction is not a perfect science. Confirm your findings before sharing them. Joseph Bell once wrote: ‘From close observation and
deduction you can make a correct diagnosis of any and every case. However, never neglect to ratify your deductions.’