The article investigates the way in which Peter Mittelstaedt has been con- tributing to the philosophy of physics for half a century. It is shown that he pursues a path between rationalism and empiricism in the sense of Erhard Scheibe's philosophy of the physicists. Starting from Kant's a priori he gives a rational reconstruction of the conceptual revolutions of 20th century physics. The central topic of his philosophy of physics is the quest for semantic self-consistency, which for quantum mechanics is a hard nut to crack.