Abstract
The filtration method for proving decidability in a focused minimal manner is a highlight of modal logic, widely used, but also posing a bit of a challenge as to its scope and what makes it tick. In this paper, we bring together a number of modern perspectives on filtration, including model-theoretic and proof-theoretic ones. We also include a few more unusual recent connections with dynamic logics of model change and logics of questions and issues. Finally, we analyze where the filtration method fails in full first-order logic, and what it still has to say there.
This paper is dedicated to Kit Fine, a prominent pioneer in mathematical modal logic. Several of his classical themes shine through in the course of what follows.
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Notes
- 1.
The valuation makes most sense if p occurs in \(\varSigma \), but we could also interpret the whole language by making the proposition letters p outside of \(\varSigma \) uniformly true or false.
- 2.
Strictly speaking, we should indicate the set \(\varSigma \) used in creating the filtrated model, but an explicit notation \(\mathfrak M ^f _\varSigma \) will not be needed, since the relevant \(\varSigma \) will usually be clear from the context.
- 3.
Double negations \(\lnot \lnot \psi \) are identified here with the original formulas \(\psi \).
- 4.
It should be pointed out that in the above proof, while for an atomic program a it is the case that \([w] R^s_a [v]\) implies that there are \(w'\in [w]\) and \(v'\in [v]\) such that \(w' R_a v'\), this may no longer be true for an arbitrary program \(\pi \). This can be shown with an easy example involving \(\pi = a^*\). Thus, in this sense, the filtrated relations correspond to the smallest filtration only for basic programs.
- 5.
In this format, filtration looks like a form of bisimulation restricted to back-and-forth behavior w.r.t. only a finite set of relevant formulas—a perspective that we will not explore further here.
- 6.
This point is due to personal communication with Clemens Kupke and Jurriaan Rot.
- 7.
- 8.
This sparse approach bypasses non-standard infinite consistent sets of formulas such as \(\{\langle a^*\rangle \varphi , \lnot p, [ a ] \lnot p,[ a ]^2 \lnot p, \ldots \}\) that do not admit of an interpretation in standard models.
- 9.
The above also shows that the finite canonical model \(W^C\) with the relations \(S_\pi \) is the least filtration of the (big) canonical model. However, as \(S_\pi \subseteq R_\pi \), the ‘real’ finite canonical model is also a filtration of the big canonical model. Condition (F1) follows from the fact that \(S_\pi \subseteq R_\pi \), and condition (F2) follows from the truth lemma for canonical models and the above Claim 3.3.5.
- 10.
The low complexity also implies that computing filtrated models can be done efficiently.
- 11.
One can also consider more purely algebraic operations. E.g., let \(f: A \rightarrow A\) be any map, with \(A_f\) the set of fixed-points of f, and set \(A\models [f]\varphi \) iff \(A_f\models \varphi \). Here A could be a Heyting algebra and f a nucleus on it, say, the double negation \(\lnot \lnot \). Then \(A_f\) is a Boolean algebra of regular elements of A, cf. Bezhanishvili and Holliday (2019). Algebraic machinery of this sort fits well with various later topics in this article, but we must leave the exploration of algebraic perspectives to another occasion.
- 12.
The cited works call this general system a ‘logic of abstraction’, emphasizing the fact that filtration stands for a very general procedure.
- 13.
The finiteness is essential for obtaining reduction axioms for the corresponding dynamic logic.
- 14.
The precise version of this procedure requires a non-trivial termination argument in terms of a syntactic measure on formulas whose details can be found in [Ilin 2018, Chap. 8].
- 15.
Here and in what follows, we suppress the explicit program notation \(\pi \) of \(\textsf{QPDL}\), since the definition of the relations is fixed for the standard filtration.
- 16.
The reader may find of interest to trace the details of this proof, and see how it literally matches the usual argument for the filtration theorem stated in the meta-language.
- 17.
A modal logic with ‘ceteris paribus’ riders expressed as arbitrary sets of formulas whose truth values are to be kept constant when comparing worlds is studied in van Benthem et al. (2009b). Its connection to the above logic of filtration remains to be understood.
- 18.
The general modal logic of bisimulation over universes of relational models seems undecidable, since bisimulation imposes a form of commutation that can encode grid structure (Blackburn et al., 2001).
- 19.
This section is discursive, putting a topic on the map, rather than presenting deep new results.
- 20.
This method involves potential exponential blow-up, and does not establish the complexity of satisfiability for \(\textsf{PAL}\), which is in fact Pspace-complete.
- 21.
Incidentally, a direct semantic filtration argument for the decidability of \(\textsf{PAL}\) is not easy to give. Could the results that follow provide a principled solution?
- 22.
Even so, we will allow arbitrary announcements of new information, without considering what happens when we restrict new information to formulas inside or close to the filtration set.
- 23.
This follows from subformula closure, or just by putting \(\top \in \varSigma \).
- 24.
This argument no longer works if we allow a richer set of dynamic-epistemic model transformations such as changing relations. We leave an analysis of this broader setting to further work.
- 25.
Notice that, since formulas can change truth values under model change, our \(\varSigma \)-dependent notion of filtration makes our functor context-dependent in a way not envisaged in van Benthem (2016).
- 26.
One option might be to relax the notion of isomorphism in our diagrams to modal bisimulation. Moreover, one should note that solutions need not be unique. Another approach would keep the update the same, but change the filtration set, as in: \(\backslash \) \(\varSigma \); \(!\varphi \) = \(!\varphi \); \(\backslash \) \(\varSigma ^{\varphi }\) with \(\varSigma ^{\varphi } = \{ \alpha | \langle \varphi \rangle \alpha \in \varSigma \}\).
- 27.
Inquisitive logics (Ciardelli et al., 2019), use generalized partitions, but this feature is orthogonal to our discussion.
- 28.
Typically, an agent will not know which issue cell she is in, until the issue has been resolved.
- 29.
- 30.
For the partitions induced by filtrations, this union will often, trivially, be the universal relation.
- 31.
One difficulty is that, after filtration by a set \(\varSigma \), filtration by an arbitrary other set \(\varTheta \) makes no sense, as the similarity type of the filtrated model has become that of the vocabulary of \(\varSigma \) only.
- 32.
Other non-valid principles include the putative equivalence of \(\backslash \) \(\varSigma \); \(\backslash \) \(\varTheta \) and \(\backslash \) (\(\varSigma \cup \varTheta \)).
- 33.
An interesting problem is determining the precise computational complexity of this notion.
- 34.
- 35.
In such models, with a variable assignment s, \(\mathfrak M, s \models \exists x \varphi \) iff there exists an object d in the domain of \(\mathfrak M\) such that (i) the assignment \(s[x:=d]\) is admissible, and (ii) \(\mathfrak M, s[x:=d] \models \varphi \).
- 36.
This is the crucial point in the proof where we use the restriction to admissible assignments.
- 37.
In particular, none of the paths \(\pi \) and \(\pi '\) needs to be a continuation of the other: they can fork off at some shared initial subpath. However, the argument about variables not changing their values, and the matching preservation of formulas in these variables also works in this extended setting, by going down to the forking point and then back up.
- 38.
The representation argument would also work, with some obvious modifications, for polyadic quantifiers \(\exists \overline{x}\) over tuples of objects. On general assignment models, these are not reducible to iterated single-variable quantifiers.
- 39.
This result can also be proved by a reduction to the Guarded Fragment in the style of Andrèka and van Benthem (1998).
- 40.
Given that, by the Janin–Walukiewicz Theorem, the \(\mu \)-calculus is the bisimulation-invariant fragment of Monadic Second-Order Logic, there may also be connections here with Fine’s early work on second-order modal propositional logic (Fine, 1970).
- 41.
The existence of a filtration method can even be seen as a criterion for simplicity of a logic.
- 42.
The results in this paper also illustrate the scope of modal logic. For instance, we noted that standard filtration fails for an undecidable system like first-order logic. But that was not the end of the story. Modal perspectives are resilient, and filtration re-emerged when we tamed complexity by extending the semantics of \(\textsf{FOL}\) to allow for dependencies between variables. As another example, consider the complexity of the meta-theory of modal logic discussed in Sect. 3.4. Working on the full meta-universe of modal models is working on a huge standard model whose complete theory can be very complex, but there could be principled reasons for restricting updates to ‘available ones’, as in the ‘protocol models’ of van Benthem et al. (2009a), thereby lowering complexity in the style of Andrèka et al. (2014).
- 43.
Super-partiality is not the only philosophical take-home point from our technical analysis. For instance, also, the quasi-models found in Sect. 3.7 are a ‘point-free’ abstraction out of standard first-order models that may fit better with certain views of the universe in current metaphysics.
References
Afshari, B., Jäger, G., & Leigh, G. (2019). An infinitary treatment of full mu-calculus. In Proceedings of WoLLIC 2019. LNCS. Springer.
Andrèka, H., van Benthem, J., Bezhanishvili, N., & Nèmeti, I. (2014). Changing a semantics: Opportunism or courage? In M. Manzano, I. Sain, & E. Alonso (Eds.), Life and work of Leon Henkin (pp. 307–337). Birkhäuser.
Andrèka, H., van Benthem, J., & Nèmeti, Ì. (1998). Modal logics and bounded fragments of predicate logic. Journal of Philosophical Logic, 27(3), 217–274.
Baltag, A., Bezhanishvili, N., Ilin, J., & Özgün, A. (2017). Quotient dynamics: The logic of abstraction. In International Workshop on Logic, Rationality and Interaction (pp. 181–194). Springer.
Baltag, A., & van Benthem, J. (2021). A simple logic of functional dependence. Journal of Philosophical Logic, 50(5), 939–1005.
Bellissima, F. (1984). Atoms in modal algebras. Mathematical Logic Quarterly, 30(19–24), 303–312.
Bezhanishvili, G., & Bezhanishvili, N. (2016). An algebraic approach to filtrations for superintuitionistic logics. In A tribute to Albert Visser. Volume 30 of Tributes (pp. 47–56). College Publications.
Bezhanishvili, G., Bezhanishvili, N., & Iemhoff, R. (2016). Stable canonical rules. Journal of Symbolic Logic, 81(1), 284–315.
Bezhanishvili, G., Bezhanishvili, N., & Ilin, J. (2018). Stable modal logics. The Review of Symbolic Logic, 11(3), 436–469.
Bezhanishvili, G., & Holliday, W. (2019). A semantic hierarchy for intuitionistic logic. Indagationes Mathematicae, 30(2), 403–469. Special issue on “L.E.J. Brouwer, fifty years later”.
Bezhanishvili, N. (2006). Lattices of intermediate and cylindric modal logics [PhD thesis]. University of Amsterdam.
Bezhanishvili, N., Ghilardi, S., & Jibladze, M. (2014). Free modal algebras revisited: The step-by-step method. In G. Bezhanishvili (Ed.), Leo Esakia on duality in modal and intuitionistic logics (pp. 43–62). Springer.
Bezhanishvili, N., & Kurz, A. (2007). Free modal algebras: A coalgebraic perspective. In T. Mossakowski, U. Montanari, & M. Haveraaen (Eds.), Algebra and coalgebra in computer science. Second International Conference, CALCO 2007, Proceedings, Bergen, Norway, August 20–24, 2007. Volume 4624 of Lecture Notes in Computer Science (pp. 143–157). Springer.
Bezhanishvili, N., & ten Cate, B. (2006). Transfer results for hybrid logic. Part I: The case without satisfaction operators. Journal of Logic and Computation, 16(2), 177–197.
Blackburn, P., de Rijke, M., & Venema, Y. (2001). Modal logic. Cambridge University Press.
Bojanczyk, M. (2002). Two-way alternating automata and finite models. In P. Widmayer, F. Triguero Ruiz, R. Morales Bueno, M. Hennessy, S. J. Eidenbenz, & R. Conejo (Eds.), Automata, Languages and Programming, 29th International Colloquium, ICALP 2002, Proceedings, Malaga, Spain, July 8–13, 2002. Volume 2380 of Lecture Notes in Computer Science (pp. 833–844). Springer.
Boolos, G. (1995). The logic of provability. Cambridge University Press.
Bull, R. A. (1966). That all normal extensions of \({\rm S}4.3\) have the finite model property. Zeitschrift fur Mathematische Logik und Grundlagen der Mathematik, 12, 341–344.
Chagrov, A., & Zakharyaschev, M. (1997). Modal logic. The Clarendon Press.
Chellas, B. (1980). Modal logic: An introduction. Cambridge University Press.
Ciardelli, I., Groenendijk, J., & Roelofsen, F. (2019). Inquisitive semantics. Oxford University Press.
Conradie, W., Morton, W., & van Alten, C. (2013). An algebraic look at filtrations in modal logic. Logic Journal of the IGPL, 21(5), 788–811.
Enqvist, S., Seifan, F., & Venema, Y. (2018). Completeness for the modal \(\mu \)-calculus: Separating the combinatorics from the dynamics. Theoretical Computer Science, 727, 37–100.
Fine, K. (1970). Propositional quantifiers in modal logic. Theoria, 36(3), 336–346.
Fine, K. (1971). The logics containing S4.3. Zeitschrift fur Mathematische Logik und Grundlagen der Mathematik, 17, 371–376.
Fine, K. (1975). Normal forms in modal logic. Notre Dame Journal of Formal Logic, 16(2), 229–237.
Fine, K. (1985). Logics containing \({\rm K}4\). II. Journal of Symbolic Logic, 50(3), 619–651.
Fitting, M. (2007). Modal proof theory. In P. Blackburn, J. van Benthem, & F. Wolter (Eds.), Handbook of modal logic. Volume 3 of Studies in Logic and Practical Reasoning (pp. 85–138). Elsevier B.V.
Gabbay, D. (1970). A general filtration method for modal logics. Journal of Philosophical Logic, 30, 323–330.
Gabbay, D. (1971a). On decidable, finitely axiomatizable, modal and tense logics without the finite model property part I. Israel Journal of Mathematics, 10(4), 478–495.
Gabbay, D. (1971b). On decidable, finitely axiomatizable, modal and tense logics without the finite model property part II. Israel Journal of Mathematics, 10(4), 496–503.
Gabbay, D. (1972). Selective filtration in modal logic. Theoria, 1(1), 29–34.
Gabbay, D. (1975). Decidability results in non-classical logics. Annals of Mathematical Logic, 8, 237–295.
Gabbay, D. (1976). Investigations into modal and tense logics, with applications to problems in linguistics and philosophy. Reidel.
Ghilardi, S. (1995). An algebraic theory of normal forms. Annals of Pure and Applied Logic, 71(3), 189–245.
Ghilardi, S. (2010). Continuity, freeness, and filtrations. Journal of Applied Non-Classical Logics, 20(3), 193–217.
Grädel, E. (2002). Guarded fixed point logics and the monadic theory of countable trees. Theoretical Computer Science, 288, 129–152.
Grädel, E., Otto, M., & Rosen, E. (1997). Two-variable logic with counting is decidable. In Proceedings 12th Annual IEEE Symposium on Logic in Computer Science, Warsaw, Poland (pp. 306–317).
Groenendijk, J., & Stokhof, M. (1997). Questions. In J. van Benthem & A. ter Meulen (Eds.), Handbook of logic and language (pp. 1055–1124). Elsevier/MIT Press.
Hansen, H. H. (2003). Monotonic modal logics. Institute for Logic: Language and Computation (ILLC), University of Amsterdam.
Hodkinson, I. (2002). Loosely guarded fragment of first-order logic has the finite model property. Studia Logica, 70(2), 205–240.
Hodkinson, I., & Otto, M. (2003). Finite conformal hypergraph covers and Gaifman cliques in finite structures. Bulletin of Symbolic Logic, 9(3), 387–405.
Ilin, J. (2018). Filtration revisited: Lattices of stable non-classical logics [PhD thesis]. ILLC, University of Amsterdam.
Kozen, D. (1988). A finite model theorem for the propositional \(\mu \)-calculus. Studia Logica, 47(3), 233–241.
Kurz, A., & Palmigiano, A. (2013). Epistemic updates on algebras. arXiv preprint arXiv:1307.0417
Leitgeb, H. (2021). A finitistic semantics for mathematics. Lecture at Munich- Konstanz Workshop on Philosophy of Mathematics/Logics, November 19–20, 2021.
Lemmon, E. J. (1966a). Algebraic semantics for modal logics. I. Journal of Symbolic Logic, 31, 46–65.
Lemmon, E. J. (1966b). Algebraic semantics for modal logics. II. Journal of Symbolic Logic, 31, 191–218.
Lemmon, E. J. (1977). An introduction to modal logic. Basil Blackwell. The “Lemmon notes”. In K. Segerberg (Ed.), Collaboration with Dana Scott. American Philosophical Quarterly, Monograph Series, No. 11.
Lutz, C. (2005). PDL with intersection and converse is decidable. In L. Ong (Ed.), Computer Science Logic, 19th International Workshop, CSL 2005, 14th Annual Conference of the EACSL, Proceedings, Oxford, UK, August 22–25, 2005. Volume 3634 of Lecture Notes in Computer Science (pp. 413–427). Springer.
Mason, I. (1985). The metatheory of the classical propositional calculus is not axiomatizable. Journal of Symbolic Logic, 50(2), 451–457.
McKinsey, J. C. C. (1941). A solution of the decision problem for the Lewis systems S2 and S4, with an application to topology. Journal of Symbolic Logic, 6, 117–134.
McKinsey, J. C. C., & Tarski, A. (1944). The algebra of topology. Annals of Mathematics, 45, 141–191.
Moss, L. (2007). Finite models constructed from canonical formulas. Journal of Philosophical Logic, 36(6), 605–640.
Pacuit, E. (2017). Neighborhood semantics for modal logic. Springer.
Segerberg, K. (1971). An essay in classical modal logic (Vols. 1–3). Filosofiska Föreningen och Filosofiska Institutionen vid Uppsala Universitet.
ten Cate, B., & Fontaine, G. (2010). An easy completeness proof for the modal \(\rm \mu \)-calculus on finite trees. In C.-H. Luke Ong (Ed.), Foundations of Software Science and Computational Structures, 13th International Conference, FOSSACS 2010 (pp. 161–175).
Tuyt, O. (2016). Canonical rules on neighbourhood frames [PhD thesis, Master’s thesis]. ILLC, Universiteit van Amsterdam.
Urquhart, A. (1981). Decidability and the finite model property. Journal of Philosophical Logic, 10(3), 367–370.
van Benthem, J. (1996). Exploring logical dynamics. CSLI Publications.
van Benthem, J. (1998). Dynamic odds & ends. Institute for Logic: Language and Computation (ILLC), University of Amsterdam.
van Benthem, J. (2010). Modal logic for open minds. CSLI Publications.
van Benthem, J. (2011). Logical dynamics of information and interaction. Cambridge University Press.
van Benthem, J. (2016). Tracking information. In K. Bimbò (Ed.), Michael Dunn on information-based logics (pp. 363–389). Springer Science Publishers.
van Benthem, J., & Bonnay, D. (2008). Modal logic and invariance. Journal of Applied Non-Classical Logics, 18(2–3), 153–173.
van Benthem, J., Gerbrandy, J., Hoshi, T., & Pacuit, E. (2009a). Merging frameworks for interaction. Journal of Philosophical Logic, 38(5), 491–526.
van Benthem, J., Girard, P., & Roy, O. (2009b). Everything else being equal: A modal logic for ceteris paribus preferences. Journal of Philosophical Logic, 38(1), 83–125.
van Benthem, J., & Liu, F. (2007). Dynamic logic of preference upgrade. Journal of Applied Non-Classical Logics, 17(2), 157–182.
van Benthem, J., & Minica, S. (2010). Toward a dynamic logic of questions. Journal of Philosophical Logic, 41(4), 633–669.
van Benthem, J., ten Cate, B., & Väänänen, J. (2009c). Lindström theorems for fragments of first-order logic. Logical Methods in Computer science, 5(3), 67–96.
Walukiewicz, I. (2000). Completeness of Kozen’s axiomatisation of the propositional \(\rm \mu \)-calculus. Information and Computation, 157(1–2), 142–182.
Zakharyaschev, M. (1992). Canonical formulas for \({\rm K}4\). I. Basic results. Journal of Symbolic Logic, 57(4), 1377–1402.
Zakharyaschev, M. (1996). Canonical formulas for \({\rm K}4\). II. Cofinal subframe logics. Journal of Symbolic Logic, 61(2), 421–449.
Acknowledgements
We thank Bahareh Afshareh, Alexandru Baltag, Balder ten Cate, Sebastian Enqvist and Julia Ilin, as well as Kit Fine and the referees and the editors of this volume, for their helpful information and constructive comments.
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van Benthem, J., Bezhanishvili, N. (2023). Modern Faces of Filtration. In: Faroldi, F.L.G., Van De Putte, F. (eds) Kit Fine on Truthmakers, Relevance, and Non-classical Logic. Outstanding Contributions to Logic, vol 26. Springer, Cham. https://doi.org/10.1007/978-3-031-29415-0_3
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