The geometry textbook written in Alexandria around 300 BCE that gave the world its most influential ancient model of the axiomatic method — truth built up by proof from a handful of stated assumptions — and remained the standard model of rigorous reasoning, and of geometry teaching, for more than two thousand years.
For roughly two thousand years, a child learning geometry in Athens, Baghdad, Paris, or Boston opened the same book. It is often said that, after the Bible, few books in the Western tradition have been printed and studied more. Yet what made Euclid's Elements matter was not the theorems, most of which he did not discover. It was the method: start with a few plain assumptions, then accept each later step only as it follows from what came before. That single idea of how to be certain shaped mathematics, philosophy, and even law for two millennia.