Finally, Popper was careful to say that it is not possible to prove that Freudianism was not true, at least in part. But we can say that we simply don’t know whether it’s true, because it does not make specific testable predictions. It may have many kernels of truth in it, but we can’t tell. The theory would have to be restated.

This is the essential “line of demarcation,“ as Popper called it, between science and pseudoscience.

Source: Karl Popper: What Makes a Theory Scientific

Another succinct article from Farnam Street.