Page "Soundness" Paragraph 15

Most proofs of soundness are trivial.

For example, in an axiomatic system, proof of soundness amounts to verifying the validity of the axioms and that the rules of inference preserve validity ( or the weaker property, truth ).

Most axiomatic systems have only the rule of modus ponens ( and sometimes substitution ), so it requires only verifying the validity of the axioms and one rule of inference.