 These two manners of speaking correspond to two different levels. On the first, linguistic level, we deal with sentential forms, i.e., expressions containing signs for variables, which become sentences (and express propositions) after appropriate substitutions are made. The second level is the level of propositions and ideas in themselves, the level of meaning. Here, Bolzano cannot use variables, letters or other indeterminate signs, for, in the realm of the propositions and ideas in themselves, there are no indeterminate entities that would correspond to sentential forms; there are only propositions, true or false. This might be the reason for Bolzano’s cumbersome way of speaking about “the idea Caius considered variable”.