Falsifiability, not confirmation, is the mark of a scientific theory

Popper's signature claim is that the traditional picture of science, building up confidence in a theory by piling up confirming observations, gets the logic backward. Confirmations are cheap and easy to find for almost any theory if you go looking for them, which is why astrology and various pseudo-psychological theories can point to seemingly endless supporting anecdotes without ever earning genuine scientific credibility. What actually distinguishes a scientific theory, Popper argues, is that it makes specific predictions bold enough to be wrong, predictions that would be contradicted by some conceivable observation if the theory were false.

A theory that forbids nothing, that is compatible with literally any possible outcome, tells us nothing about the world regardless of how much apparent evidence its supporters gather for it. Popper treats this forbidding quality, the willingness to stake a real claim that could be disproven, as the defining virtue a theory must have before it can even be evaluated as true or false in the ordinary scientific sense.

Takeaway: a theory that can explain away any possible result isn't powerful — it's empty.