Reasoning with Relevant Epistemic Logics
2025, Reasoning with Relevant Epistemic Logics, PhD Dissertation, Scuola Normale Superiore
Abstract
The present dissertation investigates the relationships between classical and nonclassical logics within a common logical platform, in which it is possible to address philosophical questions. More specifically, the focus of the dissertation lies in applications of relevant logics to problems in formal epistemology, such as the problem of logical omnisicence, the distinction between explicit and implicit beliefs, and the modeling of belief revision. As the main contribution of the present work, I devise so-called contextual modal logics, i.e. modal logics where modal operators individuate the range of application of a given logic, to be chosen from the class of extensions of weak relevant logics. In this way, I argue that it is possible to adequately formalise the context-sensitivity of reasoning, a feature which arises when we confront reasoning about the factual and the epistemic domain. Contextual modal logics are presented both semantically and proof-theoretically, by means of Hilbert-style axiom systems. The language of the logics is modularly extended, from the basic relevant language, so as to include topic sensitive operators, implicit belief operators and dynamic operators. Characterisation results are obtained for all extensions of the logics in the contextual modal family.
FAQs
AI
What are the challenges of logical omniscience in epistemic logic?
The dissertation identifies that standard modal logics suffer from logical omniscience, as agents may have knowledge resulting in incorrect beliefs regarding logical truths. This discrepancy is highlighted through examples showing contradictory mathematical propositions believed by ordinary agents.
How do contextual modal logics address logical closure properties?
Contextual modal logics propose a framework where closure principles are regulated by relevant logic, rather than classical logic, enabling a more nuanced epistemic competence. This approach allows for the incorporation of cognitive limitations in belief systems.
In what manner does relevant logic support epistemic reasoning?
Relevant logic provides a flexible framework for modeling epistemic reasoning, addressing closure properties that fail classic standards of rationality, especially concerning ordinary cognitive limitations in belief aggregation and combination. For instance, belief in implications can fail if relevant contexts are ignored.
What is the significance of the Routley-Meyer semantics in epistemic logics?
Routley-Meyer semantics allows for the interpretation of non-classical logical relationships, emphasizing weak closure properties suited for modeling epistemic reasoning. This framework proves vital in differentiating between logical states and non-logical states of information.
How do topic-sensitive logics contribute to overcoming logical omniscience?
Topic-sensitive logics regulate epistemic closure principles by imposing requirements on topic inclusion across beliefs, thereby preventing agents from making equivalent substitutive inferences under conditions leading to logical omniscience. This addresses cognitive limitations faced by agents when updating beliefs.
References (248)
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- Lemma 3.2 (C.L-heredity). For all M ∈ C.L and all φ ∈ L Rel : φ M ∈ S(↑).
- Proof. The result is established by virtually the same argument of Lemma 2.1. Lemma 3.3 (C.L-flatness). For all M ∈ C.L, all w ∈ W and all φ, ψ ∈ L Rel : 1. M, w |= t;
- M, w |= ¬φ iff M, w ̸ |= φ;
- M, w |= φ ⊗ ψ iff M, w |= φ ∧ ψ;
- M, w |= φ ⊕ ψ iff M, w |= φ ∨ ψ;
- M, w |= φ → ψ iff (M, w |= φ =⇒ M, w |= ψ);
- M, w |= φ ← ψ iff (M, w |= ψ =⇒ M, w |= φ).
- Proof. We distinguish cases, where Item 1 holds trivially by W ⊆ L.
- The following chain of equivalences holds: w |= ¬φ iff w * ̸ |= φ iff (by Condition
- =⇒ ). Assume w |= φ ⊗ ψ. Hence, there are s, t ∈ S such that Rstw, s |= φ and t |= ψ. By Condition (W3) we infer that either s = 0, t = 0 or s ≤ w. By s |= φ and t |= ψ we infer that s ̸ = 0 and t ̸ = 0, as otherwise they would contradict Lemma 3.
- Hence, s ≤ w. Similarly, by Condition (W4) we infer that either s = 0, t = 0 or t ≤ w. By s ̸ = 0 and t ̸ = 0 we infer that t ≤ w. By s, t ≤ w, s |= φ, t |= ψ and Lemma 3.2 we conclude that w |= φ and w |= ψ. (⇐). Assume w |= φ and w |= ψ. By Condition (W2) we infer that there are s, t ∈ S, namely s = t = w, such that Rstw, s |= φ and t |= ψ. We conclude that w |= φ ⊗ ψ.
- =⇒ ). Assume by contraposition that w ̸ |= φ and w ̸ |= ψ. By Condition (W1) and (W2) we infer that there are s, t, ∈ S, namely s = t = w, such that Rwst s * ̸ |= φ and t ̸ |= ψ. We conclude by Condition (W1) that w ̸ |= φ ⊕ ψ. (⇐). Assume w |= φ or w |= ψ. In order to show that w |= φ ⊕ ψ, assume for some arbitrary s, t ∈ S that Rwst. We distinguish cases. (1) If w |= φ, by Rwst and Condition (W5) we infer interested reader is referred to [152, Ch.2] for further discussion on the inferential grounds on which to accept disjunctive syllogism.
- ♢ -□φ ⊢ φ;
- □ I φ ∧ □ I ψ ⊢ □ I (φ ∧ ψ);
- Proof. Items 1 and 4 are easy consequences of Proposition 7.3.6, while Item 2 and 3 are instances of Proposition 7.3.1 and 7.3.2, respectively. Item 5 is an easy con- sequence of (FDE c 7) and Proposition 7.3.4. Items 7-9 are easy consequences of Proposition 7.3.9. Finally, item 6 is established as follows: 1. □φ 1 ∧ • • • ∧ □φ n ⊢ FDE c □ I ψ;
- Proof. (1) =⇒ (2). By contraposition, assume (2) does not hold with counter- model ({s}, V ). Then, take the FDE c -model M = ({s}, W, * , Q, V ) such that W = {s} and R = {(s, s)}. Clearly, we have that M, s |= □φ 1 ∧ • • • ∧ □φ n , since by assumption M, s |= φ 1 ∧ • • • ∧ φ n . However, M, s ̸ |= □ I ψ, since by assumption M, s ̸ |= ψ (note that this is possible only because φ 1 , . . . , φ n , ψ are propositional formulas). Hence, we conclude by Theorem 7.8 that (1) does not hold. (2) =⇒ (1).
- Assume (2). Hence, □ I φ 1 ∧. . .∧□ I φ n ⊢ FDE c □ I ψ (otherwise from the counter-model (S, W, * , Q, V ) to the latter we can extract the counter-model (W, V ) to the former). By Proposition 7.4 (item 5), □φ 1 ∧ . . . ∧ □φ n ⊢ FDE c □ I φ 1 ∧ . . . ∧ □ I φ n . Hence, we conclude that (1) holds. While classical behavior can be forced for propositional variables, this does not apply to formulas containing strict implication, as the following counter-example to a modal version of Proposition 7.5 shows. Proposition 7.6 (Modal closure failure). The following hold: 1.
- ⊤ ⊢ K (□¬(p ∧ ¬p));
- □⊤ ̸ ⊢ FDE c □ I □¬(p ∧ ¬p).
- Proof. It is immediate to verify that (□¬(p ∧ ¬p)) is valid in every Kripke model, as p ∧ ¬p is true at no state in the model. Hence, by completeness of K with respect to K-Kripke models we infer (1). Then, consider the FDE c -model M = (S, W, * , Q, V ) such that S = {s, t, u}, W = {t} Q = {(s, t), (t, u)}, s * = u and t * = t, V (p) = {s, t}. which is established by induction on the complexity of χ ′ . The base case, when
- ′ ∈ At holds by setting φ ′ := χ ′ . The induction step cases where the main connective of χ ′ is either ¬, ∧, ∨, →, □, □ L are similar, hence we show only the case χ ′ := □ξ as an illustration. By □ L -(FE [] 8) we infer that ⊢ L [ψ ′ ]□ξ ↔ □(ψ ′ → [ψ ′ ]ξ). By induction hypothesis we infer that there is γ ∈ L RRel such that ⊢ L [ψ ′ ]ξ ↔ γ, which implies by ⊢ L [ψ ′ ]□ξ ↔ □(ψ ′ → [ψ ′ ]ξ), together with □ L -(FE15) and □ L -(FE10), that ⊢ L [ψ ′ ]□ξ ↔ □(ψ ′ → γ). We conclude by setting φ ′ := □(ψ ′ → γ). Finally, the case χ ′ := [ξ]γ is ruled out by χ ′ ∈ L RRel . The following propositions show the closure properties of >. For one, the Ramsey conditional is hyperintensional in both its antecedent and consequent position. This marks a contrast with CK and ConR, where e.g. ψ ↔ χ ⇛ (φ > ψ) → (φ > χ) is admissible. By Proposition 8.4, however, a restricted version of the equivalence rule holds for > in C.F.E [] .
- Proposition 8.4 (>-Relevant equivalence). The following rules are derivable in C.F.E [] :
- □ L (φ ↔ ψ) ⇛ (φ > χ) ↔ (ψ > χ);
- □ L (φ ↔ ψ) ⇛ (χ > φ) ↔ (χ > ψ).
- Proof. The following derivations establish the result. Antecedent Restricted Equivalence (1) □ L (φ ↔ ψ) (2) □ L (□χ ↔ □χ)
- □ L -(FE [] 1)
- □ L -(FE [] 1)
- □ L (□φ ↔ □ψ) □ L -(FE [] 1) (1)
- □φ ↔ [χ]□ψ (BR), (4)
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