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## Comments on an Opinion
Categorial proof theory I am glad Wilfrid Hodges took in [9] an interest in my philosophical paper [3], but I am sorry his reading of it is marred by misunderstanding, and leads to imprudent reproaches. I don't find Peter Schroeder-Heister's ideas are evaluated correctly in [9], but I will make comments only on what is said there, in the last few pages, on my paper. In the middle of Sect. 1.3 of [9], where the critique of my paper starts, Wilfrid says that “at the heart of their [Peter's and my] arguments against 'model-theoretic semantics' is the question what can be defined in terms of what”. I was unaware that I was producing arguments against model-theoretic semantics, and as much unaware that I was dealing with Wilfrid's question. Concerning the arguments against modeltheoretic semantics, the dogmas (assumptions that everybody makes, and nobody calls into question) discussed by Peter and me are accepted not only in that kind of semantics, but also in proof-theoretic semantics, and my paper discusses their acceptance in the later kind of semantics. Concerning defining, at two places (in Sects. 5 and 6 of [3]) I mentioned the inductive definition of derivations and codes for them, and the definition of inference, i.e. deduction (as I said in Sect. 2 of [3], I used the word “inference” to accord with Prawitz's usage), as an equivalence class of derivations. Elsewhere, I spoke of Wilfrid seems to think that speaking of In a different register, in the order of I argue in [3] and elsewhere that the notion of inference should not be understood as the notion of consequence Wilfrid finds after (1.9) that a notation for derivations (does he mean by that the same as I mean by I argued at length against psychologism concerning inference towards the end of Sect. 4 of [3], and also in Sects. 6 and 7. In Wilfrid's remarks after (1.12) in [9], though at the end of the paragraph he admits that “a psychological analysis of 'making an inference' is not the right way to go”, there are still psychologistic tones in his mentioning that “people can perform an act called making an inference”. So I am afraid that the point Wilfrid ascribes to me with approval is not exactly mine. If one understands inference psychologistically, as much as Wilfrid seems to do, that point may be acknowledged, but I don't think it has much worth from a technical, proof-theoretical, point of view. To speak of impersonal inferences, not performed as an act, not made by anybody, need not be natural. This may be something in the technical language of prooftheorists. The task of proof theory however is not to stick to ordinary language, but to speak about mathematical structures involved in deduction. One finds the very interesting and important partial algebras in question in categorial proof theory (see the elementary talk [2]). Model theory, as it was conceived up to now, is blind for their logical role. Wilfrid's indignant remarks where I am accused to be saying that “the vast mass of twentieth century researchers in philosophy of logic and language all make a mistake not far short of adding 2 to 4 and getting 11” seem to stem from his assuming that a Kosta made of straw is accusing the philosophers interested in logic and language of confusing a psychologistic inference with a non-psychologistic consequence. Without putting psychologism into the picture, it was already shown by Gentzen that from a purely technical point of view it is worth studying inference syntactically, though Gentzen's sequents could be read as consequence, i.e. a generalized implication (this is how, for example, Church read them). Without psychologism, the difference between inference and consequence becomes mathematically even clearer in categorial proof theory, where one studies identity of inferences. Gentzen did not study that (though one may perhaps take that his results are pointing in that direction). I believe that at least 95 % of logicians, and 99 % of philosophers of logic and language, do not care about the codes of inferences and identity of inferences formalized by systems of equations between these codes. They are quite happy with having inferences that amount to consequence The primacy of propositions over deductions, i.e. of asserting over deducing, is in the order of explaining how language functions, and is of the same kind as the primacy of asserting over naming, which Dummett speaks about in Chap. 1 of [6], and which is mentioned in Sect. 2 of [3]. Dummett's words are: “Frege's account, if it is to be reduced to a slogan, could be expressed in this way: that in the order of One should first realize that in accordance with what was said about primacy and defining at the beginning of this note, and contrary to what it seems Wilfrid would have in the wake of Aristotle, this is not simply a matter of defining the notion of deduction in terms of the notion of proposition, or vice versa, or defining the notion of proposition in terms of the notion of name, or vice versa. The literature that would supply what Wilfrid says he has not seen or heard could start with that reference to Dummett and continue with the references to Frege [7] and Wittgenstein ([10] and [11]), which are also in [3]. It is surprising that after starting so auspiciously, on the shoulders of giants such as these last two, we don't manage to end up in the mainstream semantic literature. I agree however with Wilfrid that model-theorists usually do not care about philosophical questions concerning meaning. I don't think this is because they have superior knowledge, but because together with interest they lack knowledge about these philosophical matters—as well as knowledge about many interesting and important mathematical matters of logic not in their realm.
1. Došen, K.: Logical consequence: a turn in style. In: Dalla Chiara, M.L. et al. (eds.) Logic and Scientific Methods, Volume I of the 10th International Congress of Logic, Methodology and Philosophy of Science, Florence 1995, pp. 289-311. Kluwer, Dordrecht (1997). mi.sanu.ac.rs/kosta/publications.htm 2. Došen, K.: Algebras of deductions in category theory. In: Jokanovic´ et al. (eds), Third Mathematical Conference of the Republic of Srpska, Proceedings, Trebinje 2013, Zbornik radova, vol. I, pp. 11-18. Univerzitet u Istocˇnom Sarajevu, Fakultet za proizvodnju i menadžment, Trebinje (2014). mi.sanu.ac.rs/kosta/DosenAlgebrasofDeductions.pdf; mk.rs.ba/wp-content/uploads/2015/02/TOM1-Copy.pdf, pp. 1–8 mi.sanu.ac.rs/ kosta/publications.htm 3. Došen, K.: Inferential semantics. In: Wansing, H., (ed.) Dag Prawitz on Proofs and Meaning, pp. 147-162. Springer, Cham (2015). Preprint of 2012: mi.sanu.ac.rs/kosta/ publications.htm 4. Došen, K., Petric´, Z.: Weak cat-operads (2010). Preprint v. 8: arXiv.org 5. Došen, K., Petric´, Z.: Graphs of plural cuts. Theor. Comput. Sci. 484, 41-55 (2013). arXiv.org 6. Dummett, M.A.E.: Frege: Philosophy of Language. Duckworth, London (1973) 7. Frege, G.: Die Grundlagen der Arithmetik: Eine logisch mathematische Untersuchung über den Begriff der Zahl. Verlag von Wilhelm Koebner, Breslau (1884) (English translation by J.L. Austin: The Foundations of Arithmetic: A Logico-Mathematical Enquiry into the Concept of Number, 2nd revised edn, Blackwell, Oxford, 1974) 8. Harary, F.: Graph Theory. Addison-Wesley, Reading, Mass. (1969) 9. Hodges, W.: A strongly differing opinion on proof-theoretic semantics? In: Piecha, T., Schroeder-Heister, P., (eds.) Advances in Proof-Theoretic Semantics. Springer, Berlin (2015). This volume 10. Wittgenstein, L.: Logisch-philosophische Abhandlung. Annalen der Naturphilosophie 14, 185262 (1921) (English translation by C.K. Ogden: Tractatus logico-philosophicus, Routledge, London, 1922, new translation by D.F. Pears and B.F. McGuinness, Routledge, London, 1961) 11. Wittgenstein, L.: Philosophische Untersuchungen. Blackwell, Oxford (1953) (English translation by G.E.M. Anscombe: Philosophical Investigations, fourth edition with revisions by P.M.S. Hacker and J. Schulte, Wiley-Blackwell, Oxford, 2009) |

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