Financial theory tells us you'll never find a £10 note on the street, because the first person to walk past it would see it and pick it up. In reality, many people will simply walk straight past the money, either too busy or too focused elsewhere to notice.
Therein lies one of the core principles of value investing - the idea that it is possible to spot opportunities that others may have missed simply by looking just a little harder. Financial market boffins tell us that value can't exist, on the basis that at any point in time all the available information has been assimilated into prices, making it impossible to beat the market – the so-called efficient market hypothesis.
Even the godfather of value investing, Benjamin Graham, questioned his own methods towards the end of his career. "I am no longer an advocate of elaborate techniques of security analysis in order to find superior value opportunities," he said in an interview shortly before his death in 1976. "In the light of the enormous amount of research now being carried on, I doubt whether in most cases such extensive efforts will generate sufficiently superior selections to justify their cost."