"My freedom will be so much the greater and more meaningful the more narrowly I limit my field of action and the more I surround myself with obstacles. Whatever diminishes constraint diminishes strength. The more constraints one imposes, the more one frees one's self of the chains that shackle the spirit."
Igor Stravinsky, Poetics of Music
The other is a quote from Rosanne Cash from that recent NYT article:
In an interview, the singer Rosanne Cash said the experiments showed that beautiful compositions and technically skilled performers could do only so much. Emotion in music depends on human shading and imperfections, “bending notes in a certain way,” Ms. Cash said, “holding a note a little longer.”
She said she learned from her father, Johnny Cash, “that your style is a function of your limitations, more so than a function of your skills.”
“You’ve heard plenty of great, great singers that leave you cold,” she said. “They can do gymnastics, amazing things. If you have limitations as a singer, maybe you’re forced to find nuance in a way you don’t have to if you have a four-octave range.”
A first approximation of the difference between a music educator and a music therapist might be to say that the educator is concerned with the student being able to play in whatever style the composer asks for, while the therapist helps the client find the style most suited to that particular client's personality and abilities.