For some reason, I've seen several recent questions that can be answered by simply pointing to Newton's 3rd law (Action and Reaction). Then I stopped to think. The first law is observational. We've all seen enough (sort of) practical verifications of things not stopping or refusing to start. So I think we can say it's an observed fact about the universe. But the 3rd seems to me to be fundamentally unverifiable. It's a sort-of err... fact. Does that mean it has to be downgraded to an assumption? Newton's first assumption.