Claudio Abbado wrote this text for the first edition of 'Wahrung der Gestalt' in 1979:
I studied with him between 1956 and 1958 (together with Zubin Mehta, Borislav Pascan, Rainer Brock and others) and will never forget what he has done for all of us, not just as a teacher. He always found time for us students also outside of our studies and helped us wherever he could. His extraordinary, universal education and his great language skills allowed a unique contact. His fanatical honesty towards the score and his deep knowledge, perseverance and patience in explaining the works remain unforgettable. He always tried to show new ways to analyze a score. He wanted to be understandable for everyone and was tireless in the search for new methods. He could also treat his disciples with relentless and extremely uncomfortable irony, which we later often recognized as justified. I remember how happy he was when he came back from a tour and found the score of a work I had to conduct precisely annotated with all the comments he had taught us. Each of us then moved in a different direction, each one has developed his personal style, but the musical base has been preserved by all. Whenever I returned to Vienna after completing my studies, he found time to come to my rehearsals and concerts and to give me advice over and over again. Whether his words were positive or negative, they were always constructive. I will always remember him with great gratitude."