This, written by Carl Flesch, the Great Violin Pedagogue:
"The cases in which a person mistrusts the potency of his own expressional capacity are extremely rare in comparison with those of imaginary technical insufficiency. This is connected with an over-estimation of the technical moment, one from which, incidentally, nearly all professional violinists suffer.
Technique and expression have the same relationship to each other as has the human body to the soul which dwells within it.
It is quite indubitable that the two extremes -- neglect or over-estimation of the technical moment -- are the rule among violinists, and that the golden middle road is seldom found.
The teacher who carries exactitude to the extreme of regarding every technical mishap as a crime, is in danger of encouraging the pupil in the erroneous idea that he really has committed a crime against the spirit of music, when he drops a few notes while playing. A single unsuccessfully attempted run, under such circumstances, is enough to spoil the player's whole evening. […] A small dose of irresponsibility and a large portion of daring will provide the most appropriate mood for a day of concert performance.
The false pride of immaculate technical perfection, furthermore, leads to exaggeratedly meticulous technical studies, which it would be possible to conquer only in practice hours extended to an exaggerated degree -- while neglecting the spiritual values of the whole. In such cases, absolute mechanization in the shape of gymnastic activity with musical accompaniment soon results; while at the same time the final goal never can be reached, because actual infallibility is impossible.
We also know that the possession of means all too perfected, especially in a tonal respect, induces a certain emotional indolence, because the stimulating moment of a hindrance to overcome is eliminated. (When Casals was once asked why he did not use an instrument which was absolutely free from objection as to its tone, he answered: "Because I want to fight for my expression.")"