A theory is scientific if it can be falsified by a clear test, not just confirmed.
Popper noticed that anyone can easily find evidence that seems to confirm their ideas, even wrong ones. What separates science from pseudoscience is that a scientific theory risks something: it clearly states what observation could destroy it. A theory that can never be proven wrong is not truly testable.
Why it matters His rule remains the standard used today to judge whether research truly counts as scientific.