Theorems, Axioms, Laws, that I find useful as fundamental building blocks for thought experiments and guiding discovery, exploration, and understanding.
Bayes' Theorem: (It's all about context)
Occam's Razor: (Fewer assumptions is more likely closer to the truth)
Godel's Incompleteness Theorem: (logical consistency and truth are not synonymous)
Heisenberg's Uncertainty Principle: (you can't know everything about anything all the time)