 Bolzano’s characterization of logical deducibility is substitutional, an approach examined only to be rejected by Tarski because it was liable to produce unsatisfactory results in cases where “the language we are dealing with does not possess a sufficient stock of extra-logical constants” (Tarski 1936 [1983]: 415). Bolzano did not face exactly the same problem, because his logic deals with ideas in themselves and not with linguistic expressions. The structural problem underlying Tarski’s criticism also did not occur to him because he assumed that for every object there is an idea in itself that represents it exclusively, a thesis that is not consistently tenable with his other free-wheeling ontological assumptions (the difficulties are discussed in detail by Simons (1987: 42) and Siebel (1996: 216–223)). Tarski, who did not make Bolzano’s assumption with respect to languages, stated his definitive formulation in terms of models, or the satisfaction of sentential functions by sequences of objects.