The theory of quantum mechanics has often been thought to show an affinity with logical empiricism: in both, observation plays a central role, and questions about what is unobservable are dismissed. However, there were also strong tensions between the logical empiricism of the Vienna Circle and implications drawn from quantum physics. In the 1920s and 1930s, many physicists thought that quantum mechanics revealed a limit to what could be known scientifically, and this opened the door to a wide range of (...) speculations, in which quantum mechanics was connected with free will, organic life, psychology, and religion—speculations in which many leading quantum physicists were engaged. Members of the Vienna Circle, such as Frank and Schlick, looked at quantum mechanics for a confirmation of their empiricist views, but they were at the same time critical about these wider implications drawn from quantum mechanics, which in their eyes were connected with broader mystical and irrational trends in society. They engaged in particular with the views of Bohr and Jordan, both of whom expressed a sympathy for logical empiricism while at the same time arguing for claims that proved hard to reconcile with the scientific world conception of the Vienna Circle. (shrink)