 Here Frege appealed to the notion of “definition,” or —presuming that definitions preserve “meaning”— “synonymy”: the non-logical analytic truths are those that can be converted to formal logical truths by substitution of definitions for defined terms, or synonyms for synonyms. Since “mice” is not synonymous with “pediatrician,” (17) is not a substitution into (16) of the required sort. We need, instead, a substitution of the definition of “pediatrician,” i.e., “doctor that specializes on children,” which would convert (16) into our earlier purely formal logical truth: