The literary hero Conan Doyle - the brilliant detective Sherlock Holmes - speaks a lot and extensively about his method of deduction. And with this, he often baffles readers familiar with basic concepts of logic. After all, deductive thinking is a conclusion leading from the general to the particular. The simplest example of this reasoning: we know about the Earth's gravity; we have a general idea that water falls down, and does not rush up; repeatedly observed the process of falling water. These general messages allow us to logically imagine what Niagara Falls (private) looks like, although we have never seen it.
But the well-known Sherlock Holmes uses completely different types of conclusions, rather, more commonly known as induction, that is, ascent from the particular to the general. According to the dirt on his boots, the detective concludes that the man arrived from the countryside, according to the patches and tags of the shoemaker, that the owner of the boots is not a rich man, and according to the railway ticket sticking out of his pocket, he concludes that he arrived in London by train. In his disclosure of crimes, the famous detective goes through such a causal chain: ashes from a cigar - a smoker - his motives - the identity of the smoker. And in the end he makes the conclusion: the criminal is Mr. X. In the case of the notorious deduction of Holmes, thoughts would go a completely different way: Mr. X is very similar to the criminal, while other people involved in this case do not. Its past is dark. He had a motive to kill the victim. At the time of the crime, he has no alibi. Consequently, the killer is Mr. H.
So what deductive method does Holmes use in solving a crime? At first it seems that on the basis of the smallest details, he recreates the picture of the crime, as if it was being played before his eyes again. For example, in the case of the disappearance of Agra’s treasure: following the trail of a small foot with protruding fingers, the detective guesses that the person who left the mark was not tall and never wore shoes. Another mental effort, and here's to you: the culprit is the pygmy from the Andaman Islands.
It would seem that there is a pure induction - the ascent from the particular to the general (from private evidence to the general picture of the crime). Whereas the deductive method is a descent from the general to the particular. But in fact, there is no contradiction here. Holmes says: “Every life is an unbroken chain of cause and effect, and we can study the nature of this chain only by its link.” Remember the example of water and Niagara Falls? Here is another important quote from Conan Doyle, where the literary hero speaks of his method this way: “All crimes reveal a great tribal similarity. They (agents of Scotland Yard) introduce me to the circumstances of this or that matter. Knowing the details of 1000 cases, it would be strange not to solve the 1001st. ”
Thus, the Holmes deductive method involves knowledge of the main crimes (for example, murder, theft, forgery). His murders are classified according to the “family tree” as murder out of jealousy, for profit, out of revenge, etc. It later turns out that murder for the sake of inheriting the duke and murder committed for the possession of the Esquire’s inheritance also have their own specifics, and so on, down to the last detail. The detective, or rather, the author, being an Englishman and having an idea of island (i.e., accepted in the British Isles) case law, proceeds from the fact that the new, as yet unsolved crime had a precedent in the past, and it just needs to be fit this form.
We can say with confidence that, despite the external induction, Holmes in his logical calculations uses precisely the deductive method. Playing the violin or smoking by the fireplace, the brilliant detective ponders: what category does this crime belong to? Revenge? Jealousy? Thirst for profit? Sherlock discards everything that is inappropriate, like sifting wheat from the husk until the only correct grain is left in his hands. And he himself says about his method: "I throw aside everything impossible, and what remains is the answer to the question, no matter how fantastic it may seem."