Massimo Pigliucci in Aeon:
Popper was impressed by Einstein’s theory because it had recently been spectacularly confirmed during the 1919 total eclipse of the Sun, so he proposed it as a paradigmatic example of good science. Here is how in Conjectures and Refutations (1962) he differentiated among Einstein on one side, and Freud, Adler and Marx on the other:
Einstein’s theory of gravitation clearly satisfied the criterion of falsifiability. Even if our measuring instruments at the time did not allow us to pronounce on the results of the tests with complete assurance, there was clearly a possibility of refuting the theory.
The Marxist theory of history, in spite of the serious efforts of some of its founders and followers, ultimately adopted [a] soothsaying practice. In some of its earlier formulations … their predictions were testable, and in fact falsified. Yet instead of accepting the refutations the followers of Marx re-interpreted both the theory and the evidence in order to make them agree. In this way they rescued the theory from refutation … They thus gave a ‘conventionalist twist’ to the theory; and by this stratagem they destroyed its much advertised claim to scientific status.
The two psycho-analytic theories were in a different class. They were simply non-testable, irrefutable. There was no conceivable human behaviour which could contradict them … I personally do not doubt that much of what they say is of considerable importance, and may well play its part one day in a psychological science which is testable. But it does mean that those ‘clinical observations’ which analysts naively believe confirm their theory cannot do this any more than the daily confirmations which astrologers find in their practice.
As it turns out, Popper’s high regard for the crucial experiment of 1919 may have been a bit optimistic: when we look at the historical details we discover that the earlier formulation of Einstein’s theory actually contained a mathematical error that predicted twice as much bending of light by large gravitational masses like the Sun – the very thing that was tested during the eclipse. And if the theory had been tested in 1914 (as was originally planned), it would have been (apparently) falsified. Moreover, there were some significant errors in the 1919 observations, and one of the leading astronomers who conducted the test, Arthur Eddington, may actually have cherry picked his data to make them look like the cleanest possible confirmation of Einstein. Life, and science, are complicated.
More here.