What is the principle underpinning logic's rules aimed at avoiding contradiction? We know that contradiction is "bad", e.g. if a line of argument can be reduced to the statement A & ~A, (or if such an assertion can be extracted from an argument) the argument may be invalid. Where does this principle that contradiction leads to invalidity come from? Is it a "just because" axiom? Is it from overwhelming empirical observation? (I've certainly never seen something that both is and isn't at the same time.) More broadly (if this isn't too much) what is the relationship between basic concepts in epistemology (non-contradiction and cause/effect) and axioms of logic? What metaphysical connections or bindings exist between axioms of logic, epistemology, and objective reality? Thanks