We have all read Sherlock Holmes and marvelled at the methods he himself describes as deductive. After all, did Conan Doyle not title Chapter II of his hero's first adventure, A Study in Scarlet, "The Science of Deduction"? But what is really going on? Is the reasoning of the king of (private) detectives truly deductive?
To answer these questions, a little history may help. The teaching of philosophy in the Middle Ages, steeped in religion, followed the principles of scholasticism (a particular form of idleness, since σχολ? means "leisure" in Greek), with the avowed aim of finding common ground between revelation and reason, reconciling Christian theology with Greek philosophy, particularly that of the eclectic Aristotle. Reasoning relied especially on syllogisms and drew on the writings of ancient authors, as though every truth had to be contained in old books. The principle of syllogistic reasoning is well known: starting from a general premise and considering a particular case that may be regarded as a second premise, we deduce that what is true in the general case is also true in this particular case. Larousse provides a simple, concise formulation: "If every B is A and every C is B, then every C is A."
This type of reasoning exemplifies deduction. Once again, the Petit Larousse defines deduction very precisely: "An intellectual process that begins with premises and arrives at a conclusion."
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Deductive reasoning invents nothing! -----------------------------------------