Friday, January 10, 2014

SHERLOCK HOLMES ON THE ART OF DEDUCTION--SYSTEMATIZED COMMON SENSE

When I wrote my blog post comparing Johnny Lee Miller's Elementary to Benedict Cumberbatch's Sherlock, I had no idea that it would prove to be such a popular post. It seems that the public fascination with Sherlock Holmes has lived from the 19th century into the 21st. My fascination with the character has continued unabated since I first discovered my grandmother's two volume set of Holmes stories back in the 1950's. I had not been long in my career as a prosecutor when I began to realize that the stories were more than entertaining--they had an educational element which could inform the investigation of crimes. Several times over the years I have gone through the Holmes corpus and copied down nuggets of wisdom, and each time I managed to lose my list. Finally, a few years back I got a Kindle edition of the entire Holmes corpus and went through it highlighting the great detective's aphorisms. My list is now more or less permanent. I had a thought of going through them, systematizing them, and making comments on how they informed the investigation and prosecution of some of my cases, and perhaps I'll do that some day. For now I am mired in two other book projects, and while the commentary on Holmes remains on my bucket list it will be a long time before I can get to it--if ever.

For now, I am going to simply blog the quotes without any rhyme or reason other than to place them in the order that they appear in my Kindle edition of the Holmes corpus. On the face of them, some seem to be contradictory. The contradictions however lie in Arthur Conan Doyle's haphazard choice of words and in the ambiguities of the English language. Others among the aphorisms I deem incorrect, but this list does not compile my thoughts on the art of deduction. It compiles the thoughts of the great detective Sherlock Holmes (as they were placed in his brain by his creator, Arthur Conan Doyle).


From a drop of water a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other.

It is a capital mistake to theorize before you have all the evidence. It biases the judgment.

There is nothing new under the sun. It has all been done before.

They say that genius is an infinite capacity for taking pains. It’s a very bad definition, but it does apply to detective work.

When a fact appears to be opposed to a long train of deductions, it invariably proves to be capable of bearing some other interpretation.

The most commonplace crime is often the most mysterious because it presents no new or special features from which deductions may be drawn.


Eliminate all other factors, and the one which remains must be the truth.

I never guess. It is a shocking habit—destructive of the logical faculty.

It is of the first importance not to allow your judgment to be biased by personal qualities.

When you have eliminated the impossible, whatever remains, however improbable, must be the truth.


If … we are dealing with forces outside the ordinary laws of Nature, there is an end of our investigation. But we are bound to exhaust all other hypotheses before falling back upon this one.

The temptation to form premature theories on the basis of insufficient data is the bane of our profession.

There should be no combination of events for which the wit of man cannot conceive an explanation.


It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.

As a rule the more bizarre a thing is  the less mysterious it proves to be. It is your commonplace, featureless crimes which are really puzzling.

The bigger the crime the more obvious, as a rule is the motive.

It has long been an axiom of mine that the little things are infinitely the most important.

Singularity is almost invariably a clue. The more featureless and commonplace a crime is, the more difficult it is to bring it home.

Circumstantial evidence is a very tricky thing. It may seem to point very straight to one thing, but if you shift your own point of view a little, you may find it pointing in an equally uncompromising manner to something entirely different.

The ideal reasoned would, when he had once been shown a single fact in all its bearings, deduce from it not only all the chain of events which led up to it but also all the results which would follow from it. 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 to accurately state all the other ones, both before and after.

We will take it as a working hypothesis for want of a better.

It is, of course, a trifle, but there is nothing so important as trifles.

I had come to an entirely erroneous conclusion which shows how dangerous it always is to reason from insufficient data.

Circumstantial evidence is occasionally very convincing, as when you find a trout in the milk, to quote Thoreau’s example.

Q: If his purpose were innocent, why did he not say so? A: And if it were guilty, why did he not invent a lie? His silence appears to me to cut both ways.

It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth.

Data! Data! Data! I can’t make bricks without clay.


Nothing clears up a case so much as stating it to another person.

Improbable as it is, all other explanations are more improbable still.

See the value of imagination? … We imagined what might have happened, acted upon the supposition, and find ourselves justified.

[The theory is all surmise]. But at least it covers all the facts. When new facts come to our knowledge which cannot be covered by it, it will be time enough to reconsider it.

Any truth is better than indefinite doubt.

It is as well to test everything[, even the most obvious fact].

It is of the highest importance in the art of detection to be able to recognize, out of a number of facts, which are incidental and which are vital.

I make a point of never having any prejudices, and of following docilely wherever fact may lead me.

The most difficult crime to track is the one which is purposeless.

The principle difficulty in [the case] lay in the fact of there being too much evidence. What was vital was overlaid and hidden by what was irrelevant.


[In] the realms of conjecture [even] the most logical mind may be at fault.

It is not really difficult to construct a series of inferences, each dependent upon its predecessor and each simple in itself.

Before we start to investigate that, let us try to realize what we do know so as to make the most of it, and to separate the essential from the accidental.

It is impossible as I state it, and therefore I must in some respect have stated it wrong.

One should always look for a possible alternative and provide against it. It is the first rule of criminal investigation.

Q: What are we to do with that fact? A: To remember it—to docket it. We may come upon something later which will bear upon it.

It is a capital mistake to theorize in advance of the facts.


[In the face of these incongruous facts we may devise] a temporary hypothesis. If the fresh facts which come to our knowledge all fit themselves into the scheme, then our hypothesis may gradually become a solution.

It is an error to argue in front of your data. You find yourself insensibly twisting them round to fit your theories.

All our reasoning seems to point that way. At any rate, we may take it as a hypothesis and see what consequences it would entail.

Each fact is suggestive in itself. Together they have cumulative force.

When all other contingencies fail, whatever remains, however improbable, must be the truth.

If the matter is beyond humanity it is certainly beyond me. Yet we must exhaust all natural explanations before we fall back upon such a theory as [the supernatural].

The smallest point may be the most essential.


We must look for consistency. Where there is a want of it we must suspect deception.

One drawback of an active mind is that one can always conceive alternative explanations which would make our scent a false one.

The gravest issues may depend upon the smallest things.

My simplest art, which is but systematized common sense.

When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth. It may well be that several explanations remain, in which case one tries test after test until one or other of them has a convincing amount of support.