# Sources — first-principles

> *Primary and authoritative sources for the [first-principles](../SKILL.md) skill.*

- Aristotle, *Posterior Analytics* I.2–3 — the definition of demonstration and the regress argument: chains of demonstration cannot be infinite or circular, so they terminate in indemonstrable first principles (*archai*), known by *nous*. As treated in the Stanford Encyclopedia of Philosophy, *Aristotle's Logic*, §6 "Demonstrations and Demonstrative Sciences." https://plato.stanford.edu/entries/aristotle-logic/
- Encyclopædia Britannica, *Euclidean geometry* — Euclid's five postulates and five common notions as undemonstrated starting points from which all theorems are derived; the axiomatic method as the model for scientific exposition. https://www.britannica.com/science/Euclidean-geometry
- The popular "reason from first principles vs. reason by analogy" framing is illustrative only and is **not** cited here as a foundation — by this skill's own rule, an authority is a pointer to evidence, not bedrock.