Greek geometers (e.g. in Euclid's Elements, Book 7) define a "unit" to be 1 and "numbers" (arithmoi) to start from 2. https://mathcs.clarku.edu/~djoyce/java/ele

Greek geometers (e.g. in Euclid's Elements, Book 7) define a "unit" to be 1 and "numbers" (arithmoi) to start from 2. https://mathcs.clarku.edu/~djoyce/java/elements/bookVII/bookVII.html https://mathcs.clarku.edu/~djoyce/java/elements/bookVII/book... But in practice there are a bunch of propositions/proofs where 1 is treated as a number just like any other.