the axiom

ax·i·om n.
1. A self-evident or universally recognized truth; a maxim: “It is an economic axiom as old as the hills that goods and services can be paid for only with goods and services” (Albert Jay Nock). 2. An established rule, principle, or law. 3. A self-evident principle or one that is accepted as true without proof as the basis for argument; a postulate.