Throughout the works of Conan Doyle, and the writers who have translated his works for other media, Sherlock Holmes is shown as using "deductive logic." Deductive logic reasons from the general to the particular. The bare-bones deductive argument is the syllogism "All men are mortal; Socrates is a man; therefore, Socrates is a mortal." It's amazing how often people screw this up and argue something like, "All men are mortal; Socrates is mortal; therefore, Socrates is a man," which doesn't logically follow. That would be like saying, "All men are mortal; my kid's hamster is mortal; therefore, my kid's hamster is a man." Inductive logic reasons from particular instances to general theories and is the method used to confirm scientific theories. If you observe enough apples falling from trees, you will conclude that apples always fall down, instead of up or sideways. You might then form a more general hypothesis that includes other falling bodies, like pears. Thus is the progress of science. In the annals of literature, no character is as renowned for his powers of "deduction" as the intrepid Sherlock Holmes, but the way Holmes operates is not generally by using deductive logic at all. He really uses abductive logic. First, he carefully observes the situation, then he generalizes from his prior experience, using analogy and probability.