Rather than a monolithic movement of naive empiricists, the Vienna Circle represented a discussion forum for what were sometimes compatible, sometimes conflicting philosophical approaches to empirical evidence. The Circle's protocol-sentence debate -- here reconstructed and analyzed -- provides an exceptional vantage point from which to survey the various options and choices of the participants. Author Thomas Uebel mines the diaries, letters, and notes of the group's leading philosophers to show how their ideas emerged from real-world arguments, personal relationships, and historical settings.