Why are hypotheses and theories are always tentative explanations?

Hypotheses and theories are based on observations and expectations, but can not, or have yet to, stand up to rigorous testing of experimentation and observation. Until a hypothesis is tested, it cannot be considered a definitive fact. The same is true of a theory.
Typically a theory differs from a law in that it cannot be tested to be true all the time, and is based on observations that cannot yet be repeated and adjusted to test the theory's veracity. 

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