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Pythagoras' Law?
(lemm.ee)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
I think this is straight forward. In our everyday life laws aren't derived from something else either. They just are. And OP seemed to have a hierarchical view of them as well. They were just under the assumption that a theorem could be "promoted".
I guess a real-life analogy would be a judge making a ruling under some laws. That would make a precedent in many countries but not a law. If then a new law were passed it could invalidate that precedent similar to someone disproving a theorem.