Did you ever wonder how Benjamin Graham come up with value investing?

Benjamin Graham, Warren Buffett's mentor ran a fund with assets under management of £2.5m in 1929. The crash wiped out most of that.  In the Depression he pondered the meaning of "investing" as opposed to "speculation" and wrote the very influential Security Analysis book. Warren Buffett became his student in 1950. After regaining his investors' money, between 1936 and 1956 Graham achieved average annual returns of 20%.

 
A sound investment operation means you (a) conduct thorough analysis of the company (b) aim only for a satisfactory rate of return (c) always build in a margin of safety. If any of those factors are missing you are not investing but speculating (e.g. buying on tips or inside information ignoring analysis, trying to guess short term market moves).

He realised he had been speculating prior to the Crash. The searing experience of the Crash led to the foundational value investing philosophy he developed in the 1930s.