 The class of allowable substituends is often restricted by Bolzano; in some cases, for instance, he will require that the substitutions do not “destroy the objectuality of the proposition,” that is, do not produce an objectless subject-idea. Such would be, for example, the effect of substituting [Beijing] for [Nero] in the proposition [Nero, who is a human being, is mortal]. In his discussion of the degree of validity of a proposition relative to certain ideas, too, he stipulates that no two equivalent ideas may be included in the class of possible substituends. Thus when Bolzano speaks of “variable ideas”, we should bear in mind that the specification of a variable also involves the specification of a range of possible values.