Scientific theories should be falsifiable, testable, refutable, whichever word you want to use. Popper himself noted that Freud's theories didn't seem to fit this bill. For a theory that explains anything actually explains nothing. So both Freudianism and Marxism are deemed unscientific. We are forever guessing to the truth with our conjectured theories. Any of our theories at any time could be rendered problematic by some new piece of evidence or criticism. But if nothing is certain, we could even drop our best theories at any moment. Or do we really make progress in circles? I don't think so; science does not explicitly explain what we're trying to understand.