“The world is full of obvious things which nobody by any chance ever observes.”
Holmes distinguishes mere visibility from real observation: information can be plainly present and still go unused when attention remains superficial.
Source details
The Hound of the Baskervilles, Chapter III, “The Problem,” Project Gutenberg #2852. Sherlock Holmes says that the world is full of obvious things nobody observes after inferring from Watson’s clean clothes and boots that he spent the rainy day indoors, then asks Watson to infer where Holmes himself has been. It remains fictional detective dialogue illustrating Holmes’s observational method rather than a universal scientific rule.