Euclid's Elements

Euclid's Elements is a mathematical treatise compiled around 300 BCE, foundational to geometry and mathematical proof.

The work systematically derives geometric and number-theoretic propositions from a small set of axioms and definitions.

For over two millennia it served as the standard textbook for teaching rigorous deductive reasoning: Euclidean geometry, non-Euclidean geometry (systems that reject one of Euclid's axioms).

The phrase elementary, my dear Watson owes nothing to Euclid, but elementary in mathematics often traces to this work's methodical building from first principles.