 Bolzano begins by defining attributes of of single propositions with variable parts (TS, §147). When the method of variation is applied to a proposition, he notes, three different cases may arise: either the class of (objectual) propositions obtained by substitution contains only true propositions, or it contains only false propositions, or it contains both true and false propositions. In the first case, the initial proposition is called universally valid, in the second universally invalid, in both cases relative to the specified variables. Bolzano does not give a name to the third case; such propositions could be called neutral. Here are some examples: