Karl Popper, a founder of modern philosophy of science, observed Einstein's scientific approach as testable and falsifiable, while Freud's method was not.

Karl Popper coined the term 'pseudoscience' for theories, like Freud's, that cannot be falsified, defining falsifiability as the distinguishing feature between true science and pseudoscience.

Sigmund Freud's method of proving his theory about the influence of the unconscious and the development of human behaviors made it impossible to refute, as any information, even contradictory, could be reinterpreted to serve the theory.