This paper is an almost exact transcription of the talk of the same title that I gave in Bologna in 1991, during the 25th Multiple Seminar Meeting. I have limited myself to making the few changes necessary in order to make a text intended for oral communication to a small group suitable for a written paper. Therefore there is no systematic research to be found here, but only brief mentions of various themes that originate in what may be very banal moments of our daily task, expressed in lively language and in a colloquial and confidential manner. These notes were aimed at stimulating and encouraging participation and intervention in the seminar, and are here intended to evoke memories and thoughts in their readers — this is a little like what happens in analysis, where we cannot foresee what, of the many things that we say, will remain in the mind of the analysand and accompany him in his future life.

But I have not yet finished with my introduction; in fact I feel