This led Popper to conclude that what were regarded[by whom?] as the remarkable strengths of psychoanalytical theories were actually their weaknesses. Psychoanalytical theories were crafted in a way that made them able to refute any criticism and to give an explanation for every possible form of human behaviour. The nature of such theories made it impossible for any criticism or experiment - even in principle - to show them to be false. This realisation had an important consequence when Popper later tackled the problem of demarcation in the philosophy of science, as it led him to posit that the strength of a scientific theory lies in its both being susceptible to falsification, and not actually being falsified by criticism made of it. He considered that if a theory cannot, in principle, be falsified by criticism, it is not a scientific theory.

According to Popper, a theory is scientific only is it is susceptible to what?