I would like to dedicate this paper to the memory of Louis-André Gérard-Varet. His premature death in January 2001 deprived us all of a genuinely broad theoretical economist with deep foundational interests. He was one of the few to be conversant with the intricacies of the theory of subjective probability, and he himself co-authored a formal reconstruction of the Impartial Observer Theorem (d'Aspremont and Gérard-Varet, 1991). At the time of publishing this alternative reconstruction, I cannot refrain from remembering his exceptionally active intelligence, as well as the fruitful interchanges we had.