..if you want to be truly intelligent, you must know how to do mental gymnastics; as, you see, if you want really to have a fairly strong body you must know how to do physical gymnastics. It is the same thing. People who have never done mental gymnastics have a poor little brain, quite over-simple, and all their life they think like children. One must know how to do this—not take it seriously, in the sense that one shouldn’t have convictions, saying, “This idea is true and that is false; this formulation is correct and that one is not and this religion is the true one and that religion is false”, and so on and so forth… this, if you enter into it, you become absolutely stupid.
Ref: Questions and Answers 1955