General terms:
* Axiom: A statement that is regarded as being self-evidently true.
* Fundamental: Essential and basic.
* Core: The central or most important part.
* Principle: A fundamental truth or proposition that serves as a basis for reasoning or action.
* Truism: A statement that is obviously true.
More specific terms:
* Self-evident: So clear and obvious that it needs no proof.
* Unassailable: Not able to be attacked or disputed.
* Inherent: Existing in something as a permanent and essential characteristic.
* Immutable: Unchangeable.
* Universal: Applying to all people or things.
Figurative language:
* Foundation: The basis upon which something is built.
* Bedrock: The solid rock beneath the soil, a metaphor for something very strong and stable.
The best word will depend on the specific context and what you want to emphasize.