Because in our time the belief in the concentration of the mind exists so little and is searched so little, also a lot of other evil appears as defects of self-education, above all nervousness. While one develops the will, while one lets his muscles interact with the outer life, one has to develop his nervous system by mental concentration. [….]
The human being cannot become nervous by education of his will, but by a wrong education of his will. The will education can lead to nervousness, while the human being searches it wrongly, if he wants to get around to it with some inner means that work on his mental pictures instead connecting himself with the outer world and strengthening his will with its obstacles. Thereby his will can easily become nervous.
Source: Rudolf Steiner – GA 61 – The Self-Education of the Human Being – Berlin, 14 March 1912