In my opinion discipline is a character capacity that can be controlled by our mental capacity.
ANSWER
Mental capacity speaks to our ability to reason and the decisions we make. Ones mental capacity is determined by the physical limitations of the body, the brain, and perceptions all being inherently subjective. The wisest amongst us still only has the mental capacity of the wisest human amongst us and his capacity is as limited in objective thought by the body and it's relationship to the physical environment that surrounds him as the simplest amongst us. His wisdom did not manifest itself because of his mental capacity, it was his character that led him towards wisdom. We all, even the simplest amongst us, have the mental capacity to be disciplined, it is our character that decides whether we are disciplined or not.