At some point I’d had enough of the study of the philosophy and development of probability theory, and based on someone’s idea, I almost accidentally drifted into subjects like the foundations of geometry. This led to very a specific open problem in the proof theory of logico-mathmatical systems, one I invented but couldn’t solve, but Sara Negri did (at the helm in the picture): She had taken a course in proof theory in Amsterdam in 1992, about five years earlier. Soon it became evident that her results had been believed impossible to achieve!

This event was the start of a marvelous scientific collaboration with my wife, one that led to many collaborative articles and the two next books together with Sara, “Structural Proof Theory” and “Proof Analysis” (2001 and 2011, Cambridge U.P.). With three children, we had to invent some incredibly acrobatic tricks to be able to participate in congresses. One time I kept an improvised presentation in Munich at the blackboard of one notable Herr Professor Doktor's office, with a baby in my left arm and a piece of chalk in my right hand, auf deutsch natürlich, das war aber heftig!

Sara has moved later in her work towards philosophical logic that I’ve watched from the side. I have instead done a lot of historical-philosophical research on logic and foundations of mathematics. Even so, there are still other topics for conversation than the chef’s menu for the evening (=JvP) or the children’s school matters (=SN).