Sherlock Holmes is the invention of Sir Arthur Conan Doyle. The highly logical detective had an almost uncanny ability to form deductions based on the slightest bits of evidence. Where others might see nothing more than some cigarette ash on a carpet, Holmes could identify the type of cigarette smoked and form conclusions as to who was doing the smoking. A muddy footprint might seem to offer no clues; but to Holmes, it would be an open book.
Holmes made a couple of statements about the power of reason and deduction that have been incorporated into common lore. One was advice on how to arrive at proper conclusions. Holmes said:
Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.
How simple. Just eliminate the impossible. What is missing is any idea of how to distinguish the impossible from the highly improbable. This advice is basically useless. It looks good in a fictitious story, but has no actual place in real life.
Another quotation is from A Study In Scarlet:
From a drop of water,” said the writer, “a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it.
This is nonsense. No one would ever deduce the properties of an ocean, from a drop of water. They would not understand the forces of wind and tide on huge masses of water. They would not imagine waves, or the varying colors of the ocean. They would not imagine an endless rush of a waterfall.
These comments are perfectly fine in fiction. They give Holmes a sort of omniscience that suits the stories. But they are terrible advice for real life. And yet, people quote these ideas as though they were meaningful.