principle - noun

a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct

a rule or standard especially of good behavior

a basic truth or law or assumption

a rule or law concerning a natural phenomenon or the function of a complex system

rule of personal conduct

(law) an explanation of the fundamental reasons (especially an explanation of the working of some device in terms of laws of nature)

principle - thesaurus

precept   rationale   rule