On the occasion of Carl Friedrich von Weizsacker’s 81st birthday, a colloquium was held on July 3, 1993 in Niederpocking on the Starnberg Lake. One of the topics was “The epistemological foundation of physics from Kant to von Weizsacker”. Walter Schindler and Thomas Gornitz gave the introductory speeches. I took part in the discussion which followed. Afterwards I got the impression that I had not succeeded in making my points clear. Therefore, on July 5, I wrote an explanatory letter to Prof. von Weizsacker. This was the beginning of a correspondence on the questions intimated by the title above. In addition, a private discussion between Prof. von Weizsacker and myself took place. Some readers may be interested in the answers which Carl Friedrich von Weizsacker has given to the naive questions and views of a former physicist with hardly any education in philosophy. The letters and my notes on our discussion follow in chronological order. (I am, of course, responsible for any errors in my representation of von Weizsacker’s remarks in my notes.)