• Jaroslav Peregrin, Inferentializing Semantics.
    The whole development of modern logic is marked by various forms of confrontation of what has come to be called proof theory with what has earned the label of model theory. The heart of theories of systems of formal logic are usually considered to be proofs of soundness and (in)completeness - in effect results concerning the relationship between proof-theoretically delimited theorems and model-theoretically delimited tautologies. Those founding fathers of modern logic who saw logic as primarily a matter of rules, people like Frege, Peano or Russell, articulated the most basic axiomatic systems of logic, thus putting the concept of proof on a firm foundation. Thereafter Hilbert set up proof theory as an ambitious program based on the mathematical investigation of arithmetized proofs, aimed at showing consistency, independence and completeness of axiomatic systems by the perspicuous means of elementary arithmetic, and consequently reducing mathematics to the investigation of provability of its statements (See Kreisel, 1964 and 1968). In the thirties, Gentzen (1934; 1936) showed that proof theory need not rest on the concept of axiomatic system, but put forward the systems of natural deduction and sequent calculi (which are not based on inferring sentences from sentences, but, in effect, rather instances of inference from instances of inference). Recently, proof theory has come to be understood as a wholly general investigation of proofs and proof systems, often carried out in ways that are more algebraic than arithmetical (see Buss, 1998). However, the thirties witnessed also the spectacular demonstration of limitations of proof theory - viz. Gödel's proof of the impossibility of reaching all truths of arithmetic (not to speak about more complicated systems) by means of proof theory. This led to a revival of semantic methods in logic, due especially to Alfred Tarski (1933, 1936, 1939) and his school. (A semantic strand was, to be sure, present throughout the whole development of modern logic ever before Tarski, thanks to logicians who came to logic from algebra, i.e..
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