I definitely agree. But we cannot really use (only) already-known facts to validate a theory. A theory built around such a fact will explain it by design and wo

I definitely agree. But we cannot really use (only) already-known facts to validate a theory. A theory built around such a fact will explain it by design and won’t have as much explanatory power as if the fact were verified after the theory is established. Of course if a theory cannot explain an already-known empirical fact, then it’d better provide a good explanation, like a different domain of validity. Because of this, I would not consider relativity to have been validated until things like the Kennedy–Thorndike experiment. Even if it could already reconcile the principle of relativity and a constant speed of light, which was a significant intellectual achievement. Explaining the Michelson-Morley experiment was great, even a posteriori.