What Holmes Really Did

Sherlock Holmes is famous for "deduction." But when he examines a pocket watch in The Sign of Four and concludes its owner was an alcoholic, he is not deducing at all. He is observing specific details — like the scratches around the keyhole — and inferring the most likely explanation. Real deduction goes from general rules to specific conclusions.

Deduction works like this: All mammals breathe air. Dogs are mammals. Therefore, dogs breathe air. The conclusion is guaranteed if the premises are true.