Data or theory - what should an investor emphasize?

The empiricist thinks he only believes what he sees, but he is much better at believing than at seeing.
-George Santayana

Never theorize before you have data. Invariably you end up twisting facts to set theories, instead of theories to suit facts.
- Sherlock Holmes

It is important to be focused on the numbers but it more important to have a theory or mental map of what you expect and how you see the world. Data is never enough even if you are a priced-based manager. You have to have an understanding of what causes prices to move and why prices do not adjust immediately. The need for a theory is critical for any empirical work.