Popper’s way of demarcating science from pseudo-science is to say that conjectures which cannot be criticised and tested empirically are useless, and not science. He was deeply influenced in this view by two twentieth century intellectual developments, and the contrast between them. After centuries of believing that Newtonian gravitational theory truly described the astronomical universe, scientists accepted the superiority of Einstein’s new theory of gravitation. A theory once thought to be securely founded was overthrown because it could not account for small discrepancies in the motion of the planet Mercury. Popper also saw how in stark contrast psychoanalytic theories seemed impervious to unwelcome evidence, and so he concluded that psychoanalysis was not a science.