The #1 misconception about Popperian philosophy is that his main idea was falsifiability.

People often call this philosophy 'falsificationism', when really that was a tiny facet of one part of its philosophy of science.

What was falsifiability all about?

There were theories floating around which purported to be science, but which Karl Popper thought were not science:

"What is wrong with Marxism, psycho-analysis, and individual psychology? Why are they so different from physical theories, from Newton's theory, and especially from the theory of relativity?"

It wasn't that he was interested in whether they were true, but whether they had the character of science. Those three theories "posed as science", he said, but "had in fact more in common with primitive myths than with science".

Popper called this the Problem of Demarcation: how do you distinguish science from non-science? What makes science different?

His solution to the problem of demarcation was:

Science is empirically testable. Refutable by experiment.

Here's where people get confused.

Popper was interested in this problem because he wanted to figure out what made some theories pseudo-science.

But his theory had reach – it also answered the question: what makes science different from philosophy.

Something can be non-scientific but true — it just doesn't come under the umbrella of 'science'.

Popper didn't emphasise this, because his original problem was, "Why is Marxism and psycho-analysis pseudo-science?"

So because he didn't emphasise it, everyone is like, "Aha! So everything that doesn't follow Popper's criterion of science is rubbish".

When actually, a lot of things that purport to be science are just philosophy (which may still be valuable!).

What is Popper's main theory if not falsifiability?

Karl Popper's main work was in epistemology, not the philosophy of science. (Although he came via trying to understand things in the philosophy of science.)

The basic gist of his core idea is: we have existing theories, then we criticise them, and modify them to get better theories.

Falsification is just one kind of criticism.

There are lots of other kinds: internal consistency, being contradicted by other things we know, not explaining what it purports to explain, being vague, etc.

Falsification is not even the main kind of criticism in science.