The rigorous characterization of ‘and nothing else’ is important, not just to arithmetic, but to
logic in general. For example, recall the way the syntax of propositional logic is defined. We say that
every upper case letter is a formula, that the negation of any formula is a formula, that the conjunction of
any two formulas is a formula, etc., and moreover nothing else is a formula. The usual way of
characterizing the exclusion (‘nothing else’) clause is to say that the set of formulas is the smallest set of
strings of symbols that contains all atomic formulas and is closed under the formation of negation, con-
junction, etc.