If the human being is active with gymnastics where he has to set his muscles in motion in such a way that he can infer nothing at all that he does not strain his intellect, but directly develops with the activities and not with intellectual understanding, then we deal with a self-pedagogic play.
From it, we directly gain an important principle of any self-education. This is that the human being who has to educate himself by the education of his intellect and in particular by the education of his will depends on the care of the contact and interrelation with the outside world. The human will can be educated not by inner intellectual training, but it strengthened, so that the human being has a firm hold inside if he maintains the will while the own will and the outside world interact.
That is why the usual self-education is almost injured if the human being tries to strengthen his will for the outer life by inner means, by inner training.
Source: Rudolf Steiner – GA 61 – Human History in the Light of Spiritual Investigation: The Self-Education of the Human Being – Berlin, 14 March 1912