 Popper was also profoundly impressed by the differences between the allegedly “scientific” theories of Freud and Adler and the revolution effected by Einstein’s theory of Relativity in physics in the first two decades of the twentieth century. The main difference between them, as Popper saw it, was that while Einstein’s theory was highly “risky”, in the sense that it was possible to deduce consequences from it which were, in the light of the then dominant Newtonian physics, highly improbable (e.g., that light is deflected towards solid bodies—confirmed by Eddington’s experiments in 1919), and which would, if they turned out to be false, falsify the whole theory, nothing could, even in principle, falsify psychoanalytic theories. They were, Popper argues, “simply non-testable, irrefutable. There was no conceivable human behaviour which could contradict them” (1963: 37). As such, they have more in common with myths than with genuine science; “They contain most interesting psychological suggestions, but not in a testable form” (1963: 38). What is apparently the chief source of strength of psychoanalysis, he concluded, viz. its capability to accommodate and explain every possible form of human behaviour, is in fact a critical weakness, for it entails that it is not, and could not be, genuinely predictive. To those who would respond that psychoanalytic theory is supported by clinical observations, Popper points out that