This is also the reason for the quote before the foreword:
"I know, or rather I should say that I'm told, for I am certainly unable to see it, that my classes are increasingly crowded, with more and more students attending, and that many of them are not even registered in the course. I thus think we can safely assume that they want to listen to my lectures, right?"
When he says above that many students are attending without being even registered he means that they´re attending in spite of his course not being a requirement for them. They´re attending as listeners, regardless of their earning no credit for it, out of sheer pleasure and genuine interest.
In Spanish: "Yo sé, o más bien me dicen, porque desde luego yo no puedo verlo, que mis clases se llenan cada vez más de alumnos, y que muchos no están ni siquiera inscriptos en la materia. De modo que debiéramos suponer que quieren oírme, ¿no?”