Philosophy of science
The central issue for Popperian philosophy of science was the demarcation problem: how can genuine science be distinguished from pseudo-science? Popper thought that truly scientific disciplines (such as physics) must share some common feature which is lacking in what he considered to be pseudo-sciences (such as psychoanalytic theory). Moreover, this common element must be something to do with the logical structure of the theories found in genuine sciences. Therefore, the demarcation problem is the quest for the logical feature peculiar to scientific theories. In Popper’s view, the distinctive logical fact about science is that its theories can be tested against empirical evidence. If a physical theory, for example, makes a prediction which turns out to be false, then the theory itself must be false. Psychoanalytic theory, on the other hand, cannot be empirically falsified because it is compatible with any empirical data whatsoever (so Popper argued).