"You can't overestimate the importance of psychology in chess, and as much as some players try to downplay it, I believe that winning requires a constant and strong psychology not just at the board but in every aspect of your life."

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Emotional instability can be one of the factors giving rise to a failure by chess players in important duels. Under the influence of surging emotions (and not necessarily negative ones) we sometimes lose concentration and stop objectively evaluating the events that are taking place on the board.

— Mark Dvoretsky