Daniel Kastler played for many many years a key role as a leading Mathematical Physicist in developing Algebraic Quantum Field Theory. He laid the foundations of the subject as the famous
"Haag-Kastler" axioms in his joint paper with Rudolf Haag in 1964. He gathered around him, in Bandol, a whole international school of mathematicians and physicists. With the devoted help of his beloved wife Lisle, he created a perfect atmosphere for lively discussions ranging from technical points to philosophical issues, where the quest for truth was the only goal and "arguments of authority" were banished. I remember my first encounter with Daniel and Lisle, in Seattle at the Battelle institute where I had been invited in 1971, as a young mathematician. Being a beginner I would have expected to be treated with condescendance by the "pros" of the field and was amazed and in fact enthralled by the warmth, the openness of mind which I found in this group, and with a clear maximum at "Daniel", a charming man combining a German care for exactness of details with a Mediterranean tendency for story telling. Daniel had a deeply romantic personality which came as a precious antidote to the harshness of the technical work which is unavoidable when doing research. Discussing with him, one was free to dream. He had a great sense of "metaphors" and one of his preferred one concerned the despise with which particle physicists sometimes treat the "Standard Model" which some consider just as an effective theory. What Daniel used to say is that: "the Standard Model is in fact a Shakespearian Prince disguised as a beggar, but with diamonds in his pockets!" And Daniel was also a remarkably skilled "technician" who was able, while in his seventies, to perform with utmost care the most complicated calculations needed to give solid backing to an abstract idea.
Over the years I have understood that the special warmth and openness of mind which I had met in the field already in 1971, and which I have appreciated all my life, owed a lot to the presence of Daniel among the leaders. His disparition is a great loss for our field.