Uncertainty principle
Scientists had already learned why certain substances give off characteristic colors of light when they are heated or otherwise excited. Heisenberg was trying to explain why these colors each have a characteristic brightness. It would not have been good enough if he and the other scientists had just said, "Well, that's just the way it is." They were sure that there had to be a good reason for these differences, and for the fact that the ratios among the bright line strengths were always the same for each sample of an element.