Let’s arm wrestle! To prepare, you drink a dram of Strength Potion (TM) while I drink filtered water. Then you defeat me without breaking a sweat. You won because of the potion, right?
Nope, we’d offend science itself if we jumped to that conclusion. After all, we’re different people with different arms. Since we could come up with a thousand potion-free explanations for your victory, we’d look foolish claiming we know anything at all about the potion’s efficacy.
But what if I arm-wrestled my genetic clone? She drinks the potion and I don’t. Then she wins effortlessly. Is it the potion?