Sherlock Holmes: Logician or Theseologist?
I propose to devote my declining years to the composition of a textbook which shal focus the whole art of detection into one volume.
—Sherlock Holmes in The Adventure of the Abbey Grange
He is a Logician
A logician studies the way we ought to reason; she is interested in the distinction between corect reasoning and incorect reasoning. Although we al reason and are often interested in whether our reasoning is valid we are not a l logicians because we do not make a study of it; that is, we do not reflect deeply enough on this subject.
Now Sherlock Holmes reasons a great deal—Watson cals him the greatest reasoning machine in the world. But he not only reasons he also reflects on how we should reason. Indeed, he was planning on writing a textbook on the art of detection when he retired. When a
person is planning on writing on a subject that he knows so wel in practice he must have thought deeply about the subject.
Sherlock Holmes' profession is crime detection. His expertise is in finding out what criminals are hiding from the rest of the world. But as he has demonstrated over and over again his kind of reasoning can be applied to al kinds of situations in which we want to uncover knowledge of things to which we do not have direct access. Sherlock Holmes was not present when a crime was commited, , but after he had done his work he was able to describe what happened, if not to the last detail, then at least (as he says) in essentials. Watson went out one morning unaccompanied. Afterwards Sherlock Holmes had no dificulty teling him, Watson, what he, Watson, had been doing. Sherlock Holmes is interested in the kind of reasoning that in a textbook on logic would come under the heading 'Scientfic Method'. Indeed, in textbooks as wel as on other occasions Sherlock Holmes is often brought in to ilustrate scientfic method. In one of the articles he has published Sherlock Holmes has this to say concerning the power of this kind of reasoning 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.
A Study in Scarlet
Things scientists know are not confined to things they have seen or heard. Scientists are able to tel us what happened milions of years ago—just as Sherlock Holmes is able to tel us what happened when a crime was commited. Now how is this done if not by reasoning? There are two ways for us to know: either directly or by inference. Since scientists were not there milions of years ago; since Sherlock Holmes was not there when the crime was commited; they could not have known what they know directly. They have to rely on reasoning. It is through reasoning that they find out what they want to know.
Sherlock Holmes is aware that he engages in a special form of reasoning—special when compared to the kind we ordinarily do. He cals it reasoning backwards.
In solving a problem of this sort,...