n.
(law) an explanation of the fundamental reasons (especially an explanation of the working of some device in terms of laws of nature); "the rationale for capital punishment"; "the principles of internal-combustion engines"
n.
a rule or law concerning a natural phenomenon or the function of a complex system; "the principle of the conservation of mass"; "the principle of jet propulsion"; "the right-hand rule for inductive fields"
n.
a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct; "their principles of composition characterized all their works"