John Derbyshire on National Review Online

The Importance of Not Thinking Too Much
Foundational crises.
It is conventional wisdom among soldiers, at any rate in the British army, that a man is no good for combat after the age of 35 because he thinks too much. I suppose that in the particular case of soldiering, and strictly from the actuarial, as opposed to career, point of view, you could argue that thinking too much is therefore a good life strategy. In general, however, it is a bad one. It would not do for the concert pianist to think about every note before he struck it, or for the tennis player to perform a mental exercise in mathematical ballistics before swinging the racquet, or for the courtroom lawyer to carefully ponder the pros and cons of each question before addressing it to the witness.