The mathematical system of natural numbers 0, 1, 2, 3, 4, ... is based on an axiomatic system first written down by the mathematician Peano in 1889. He chose the axioms (see Peano axioms), in the language of a single unary function symbol S (short for "successor"), for the set of natural numbers to be:
There is a natural number 0.
Every natural number a has a successor, denoted by Sa.
There is no natural number whose successor is 0.
Distinct natural numbers have distinct successors: if a ≠ b, then Sa ≠ Sb.
If a property is possessed by 0 and also by the successor of every natural number it is possessed by, then it is possessed by all natural numbers ("Induction axiom").