EntityQ1548746· pop 15· linked from 416 articlessecond-order logicSign in to saveextension of first-order logic allowing quantification over functions and relationsConnectionsfirst-order logicEntitysemantic theory of truthEntitymathematical logicEntityWillard Van Orman QuineEntityinjectionEntityGödel's incompleteness theoremsEntitysurjective functionEntitycountable setEntityPeano axiomsEntityformal systemEntityZermelo–Fraenkel set theoryEntityrule of inferenceEntityrecursive setEntityLöwenheim–Skolem theoremEntitywell-formed formulaEntitysemantics of logicEntityrecursively enumerable setEntitystructureEntitytheoryEntitydiagramEntityCategoriesCharles Sanders PeirceSystems of formal logic