 Note that a sentence need not be perfectly distinct (i.e., such that there is a one-to-one structure-preserving correspondence between its parts and the parts of the proposition is expresses) to be used in this way, provided only that the parts of the sentence that are replaced by signs for variables do correspond to parts of the proposition. For instance, if we are satisfied that ‘mammal’ and ‘bear’ correspond to parts of the proposition [Some mammal is a bear], we can use the sentential form ‘Some A is a B’ to determine a class of propositions, even if it turns out that this proposition might be more distinctly expressed by a different sentence, e.g., by ‘There exists an individual that is a mammal and that is also a bear.’