 If certain parts of a sentence correspond to parts of the proposition it expresses, the sentential forms we obtain by replacing one or more of these parts with signs for variables, along with a specification of permissible substituends, will determine not just a class of sentences but also a class of propositions of the kind just described. Bolzano sometimes calls such classes of propositions propositional forms. Strictly speaking, however, he reserves this term for the sentential forms that determine such classes: