I never attempted to convince my students to consider me infallible - only a "music teacher" needs that. On the contrary, I often risked saying something that I later had to revise - I have frequently given instructions that, when applied, proved to be incorrect and which I had to correct.

Perhaps my mistakes were not beneficial for the student, but they caused him no harm. The fact that I openly recognized my mistakes must have given him something to think about. As for me, since the instructions I gave were untested products of my own thinking, I was led by my mistakes, which were immediately exposed, to re-examine my instructions from scratch and improve their wording.

So this is how this book was created. From the mistakes my students made as a result of inadequate or incorrect instructions, I learned how to give the right guidance. The successful completion of the exercises by my students established the correctness of my efforts without leading me to the illusion that I had solved the problem definitively.

(from the book's preface)