It did not cross my mind in those days that I might, one day, be asked to contribute to illustrate certain aspects of his work. Believe me, it is not an easy task; there are my dif®culties ± well known to all those who know me ± such as never ®nding enough time to write a short paper. However, there is something else: in spite of Rosenfeld's statements, which at ®rst seem to facilitate the task for those who have to illustrate his work, his claim not to be a theoretician is, in reality, a theoretical position in its own way, which needs to be understood in its own speci®city and in the problems which arise from it.