As for how "heart" affects play, let's take one extreme example. To protect the innocent we shall call this totally hypothetical player Barry Neffield. This player would, when lacking emotional desire aka heart, be known to tank plays and play poorly to get traded to another team. This player (again, that is a pure construction to prove a point) would not do this simply once in his career, he would do it repeatedly. Because we can't quantify this player's lack of heart, does that mean it will not effect his performance?