 If Bolzano’s thesis of semantic atomism is correct, then there is a finest form for every proposition; namely, the one in which every simple idea in the proposition is considered independently variable. We can see from this the task that confronts someone who seeks to define the concept of a proposition as a certain kind of structured whole using formation rules: this would require not only a specification of all possible syntactic structures, but also a complete set of semantic categories (since different sites of variation may come with different classes of admissible items). Moreover, it would be necessary to do this not just for a formal language of our own devising, or for a single natural language, but for any possible language. Seen in this light, Bolzano’s forthright admission that he could find no satisfactory definition of the concept of a proposition is hardly surprising.