 Another special case of deducibility is logical deducibility; it holds when all the invariable elements in a deduction are logical concepts (TS, § 223). There are obvious parallels between this notion and Tarski’s notion of logical consequence. More intriguing, perhaps, are the similarities between Bolzano’s logical deducibility and a relation described by Frege in the course of his dispute with Hilbert and Korselt (Frege, 1906, 426ff), which are so striking that Göran Sundholm suggested that Frege may have profited from an unacknowledged acquaintance with Bolzano’s logic (Sundholm, 2000; but cf. Künne, 2008, 330ff.). Another plausible explanation is that Frege simply reverse-engineered something quite like Bolzanian logical deducibility by making the logical methodology of Hilbert’s Foundations of Geometry explicit.