This paper reviews the debate between Carnap and Schrödinger about Hypothesis P (It is not only I who have perceptions and thoughts; other human beings have them too)–a hypothesis that underlies the possibility of doing science. For Schrödinger this hypothesis is not scientifically testable; for Carnap it is. But Schrödinger and Carnap concede too much to each other and miss an alternative understanding: science does not depend on an explicit hypothesis concerning what other human beings see and think; it is simply a practice of communication which anticipates or presupposes the perfect interchangeability of positions amongst the members of the linguistic community. The mentalistic vocabulary of folk-psychology, used by Carnap and Schrödinger, does not take first but last place in this perspective; because it does nothing but express after the event the confidence to which the disputants bear witness regarding a generally successful practice of communication.