More important for us is his treatment of quantification. Peano allows propositions to contain variables, that is to say, he utilizes open formulas. He does not give an interpretation of open formulas. He does not tell us what they represent. But they are used in his theory of quantification. Peano only has a universal quantifier. He does not define an existential quantifier in the ‘Principles’. The quantifier is always attached to a conditional or biconditional. Quantified propositions are always of the form