Sentence:  That there is an incomprehended power in Nature, is an <WSD>axiom</WSD> to which all must assent: but what that power is must be reduced to an <WSD>axiom</WSD> likewise, before any defence of prophecy, miracle, or any kind of superstition, can be made on solid grounds. 
Meaning of the word marked with <WSD> tags:  An established principle in some artistic practice or science that is universally received.