In 1967, when I just began at Harvard, Res Jost and Klaus Hepp invited me to spend a term as Gastprofessor at the E.T.H. I went the following May to give a course on "Constructive Quantum Field Theory." Jost assigned Konrad Osterwalder (at the time a beginning graduate student) as my Assistant. Konrad wrote up the notes from my lecture course, and although we originally planned to publish a book, the field was moving so fast that we never spent our time doing other things.

Some other graduate students at the time were Sergio Albeverio, Phillipe Blanchard, Peter Minkowski, Ruedi Seiler, and Edouard Zehnder, while Jean-Pierre Eckmann and Jürg Fröhlich were undergraduates. One day I wanted to find a book in the main library, and as a Professor I was allowed to enter the "stacks." To my surprise, the book I wanted to see lay at the center of a real stack: a cube of books on the floor approximately 1.5 meters on each side! I filed a written request, and returned the next day. By then the library staff had rearranged the stack in order to remove the volume I wished to read.

I also spent time at the E.T.H. during the summers of 1969 and 1970. I especially treasure one memory from the third visit: that summer Jim Glimm and I stopped to visit the Seminar für Theoretische Physik just before staying for two months at an intense summer school in Les Houches. During that visit Res Jost hosted a little ceremony in the Hochstrasse 60 seminar/tea room where he bestowed on us both the title "Honorary Member of the Seminar for Theoretical Physics" with an open invitation to return to the E.T.H. whenever possible.

In the summer of 1969 Robert Schrader came from the E.T.H to Harvard as my first postdoctoral fellow. Konrad Osterwalder and Jürg Fröhlich followed shortly afterward. Konrad remained at Harvard until 1977. These collaborations cemented in place a relationship and exchange of students and visitors between Harvard and the E.T.H. that still continues today.

During both 1992 and 1993 I vistited the Forschungsinstitut für Mathematik in the Hauptgebaude. This had been founded in the 1960's by Beno Eckmann to foster mathematical reserach and to bring visitors to the E.T.H. Today in his 90's Eckmann still comes daily to his desk in the Forschungsinstitut. One day in 1993 as I was about to go to lunch, I read an email from Debbie Haimo that told me the previous day that Princeton mathematician Andrew Wiles had announced at a conference at the Newton Institute in Cambridge that he had proved Fermat's Theorem. As I made my way into the Dozentenfoyer I recognized a large table of mathematicians around Jürgen Moser (Director of the Forschungsinstitut at the time) with Gerd Faltings (then professor at the Institute for Advanced Study in Princeton). Faltings had come to the E.T.H. to give a lecture that afternoon. The lunch-table of mathematicians included John Mather and Sergio Kleinerman as well, two other visitors from Princeton.

So I headed directly toward the table to speak with Faltings. An interesting exchange ensued:
Arthur: "I just received an email saying that Wiles announced a proof of the Fermat Theorem. Is that be the case?"
Faltings (with disbelief): "André Weil?" (The room was packed and the background noise drowned out much conversation. So Faltings thought I mentioned the eminent, but then aging doyen of mathematics Weil.)
Arthur (more loudly): "No. Andrew Wiles."
Faltings leaned back, tilted his chair, and stared directly at the ceiling. A long interval of silence ensued. It was probably only ten or fifteen seconds, but at the time it seemed like an eternity. For Faltings had thought about Fermat's problem extensively. At the time he represented the number-one expert worldwide on that circle of ideas, having established a famous partial result with his proof of Mordell's conjecture. After the silence, the legs of Faltings chair came back to the ground. He burst out with laughter, exclaiming: "I'm sure its wrong!"
As it turned out, Faltings was right.
But about a year later Wiles found another variation of the proof with Richard Taylor, definitively solving the 350–year old Fermat problem.

A couple of years later, Moser told me that he was incredulous —as was everyone else at the table. Three Princeton mathematics professors had been in the luncheon group; how could it be possible that none of them had known about such important work being carried out in Princeton —work that must have been in progress for a long time. The lunch group had concluded that I was playing a practical joke on Faltings!