Fundamental truth is a concept that has been around for centuries. It is an idea or belief that is accepted as being true and is not subject to change. The two crossword solutions for this term are PRINCIPLE (9 letters) and AXIOM (5 letters). A PRINCIPLE is a fundamental truth or law that serves as the basis for a system of belief or behavior. An AXIOM is a self-evident truth that requires no proof or explanation. Both principles and axioms are used to explain and guide our understanding of the world.