You can never be free until you develop the mental strength to stop fretting over things you cannot control.
The more difficulties one has to encounter, within and without, the more significant and the higher in inspiration his life will be.
To create something you must be something.
I should say that mental strength is No. 1, experience [is] No. 2, physical strength is No. 3, and genetic ability that you're getting from Mother Nature probably comes after.