School of Mathematical and Computing Sciences
P.O. Box 600
This is an exposition and analysis of van Benthem's original proof, hitherto unpublished, that if the class of structures (frames) validating a modal formula is closed under elementary equivalence, then it is the class of all models of a single first-order sentence.
Photo taken by Rob Goldblatt in a Tubingen restaurant in December 1977.
From left to right are Jens Erik Fenstad, Johan van Benthem, Dagfinn Follesdal, Ede Zimmermann, Arnim von Stechow, Uwe Monnich, ??