Introduction to modal logic Luís Soares Barbosa José Proença HASLab - INESC TEC Universidade do Minho Braga, Portugal February/March 2018
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics A logic A language i.e. a collection of well-formed expressions to which meaning can be assigned. A semantics describing how language expressions are interpreted as statements about something. A deductive system i.e. a collection of rules to derive in a purely syntactic way facts and relationships among semantic objects described in the language. Note • a purely syntactic approach (up to the 1940’s; the sacred form) • a model theoretic approach (A. Tarski legacy) 2 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Semantic reasoning: models • sentences • models & satisfaction: M | = φ • validity: | = φ ( φ is satisfied in every possible structure) • logical consequence: Φ | = φ ( φ is satisfied in every model of Φ) • theory: Th Φ (set of logical consequences of a set of sentences Φ) 3 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Syntactic reasoning: deductive systems Deductive systems ⊢ • sequents • Hilbert systems • natural deduction • tableaux systems • resolution • · · · • derivation and proof • deductive consequence: Φ ⊢ φ • theorem: ⊢ φ 4 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Soundness & completeness • A deductive system ⊢ is sound wrt a semantics | = if for all sentences φ ⊢ φ = ⇒ | = φ (every theorem is valid) • · · · complete ... | = φ = ⇒ ⊢ φ (every valid sentence is a theorem) 5 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Consistency & refutability For logics with negation and a conjunction operator • A sentence φ is refutable if ¬ φ is a theorem (i.e. ⊢ ¬ φ ) • A set of sentences Φ is refutable if some finite conjunction of elements in Φ is refutable • φ or Φ is consistent if it is not refutable. 6 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Examples M | = φ • Propositional logic (logic of uninterpreted assertions; models are truth assignments) • Equational logic (formalises equational reasoning; models are algebras) • First-order logic (logic of predicates and quatification over structures; models are relational structures) • Modal logics • ... 7 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Modal logic (from P. Blackburn, 2007) Over the years modal logic has been applied in many different ways. It has been used as a tool for reasoning about time, beliefs, computational systems, necessity and possibility, and much else besides. These applications, though diverse, have something important in common: the key ideas they employ (flows of time, relations between epistemic alternatives, transitions between computational states, networks of possible worlds) can all be represented as simple graph-like structures. Modal logics are • tools to talk about relational, or graph-like structures. • fragments of classical ones, with restricted forms of quantification ... • ... which tend to be decidable and described in a pointfree notations. 8 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics The language Syntax φ ::= p | true | false | ¬ φ | φ 1 ∧ φ 2 | φ 1 → φ 2 | � m � φ | [ m ] φ where p ∈ PROP and m ∈ MOD Disjunction ( ∨ ) and equivalence ( ↔ ) are defined by abbreviation. The signature of the basic modal language is determined by sets PROP of propositional symbols (typically assumed to be denumerably infinite) and MOD of modality symbols. 9 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics The language Notes • if there is only one modality in the signature (i.e., MOD is a singleton), write simply ♦ φ and � φ • the language has some redundancy: in particular modal connectives are dual (as quantifiers are in first-order logic): [ m ] φ is equivalent to ¬� m � ¬ φ • define modal depth in a formula φ , denoted by md φ as the maximum level of nesting of modalities in φ Example Models as LTSs over Act. MOD = P Act – sets of actions. �{ a , b }� φ can be read as “ after observing a or b, φ must hold. ” [ { a , b } ] φ can be read as “ after observing a and b, φ must hold. ” 10 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Semantics M , w | = φ – what does it mean? Model definition A model for the language is a pair M = � F , V � , where • F = � W , { R m } m ∈ MOD � is a Kripke frame, ie, a non empty set W and a family R m of binary relations (called accessibility relations ) over W , one for each modality symbol m ∈ MOD. Elements of W are called points, states, worlds or simply vertices in directed graphs. • V : PROP − → P ( W ) is a valuation. When MOD = 1 • ♦ φ and � φ instead of �·� φ and [ · ] φ • F = � W , R � instead of F = � W , { R m } m ∈ MOD � 11 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Semantics Safistaction: for a model M and a point w M , w | = true M , w �| = false M , w | = p iff w ∈ V ( p ) M , w | = ¬ φ iff M , w �| = φ M , w | = φ 1 ∧ φ 2 iff M , w | = φ 1 and M , w | = φ 2 M , w | = φ 1 → φ 2 iff M , w �| = φ 1 or M , w | = φ 2 M , w | = � m � φ iff there exists v ∈ W st wR m v and M , v | = φ M , w | = [ m ] φ iff for all v ∈ W st wR m v and M , v | = φ 12 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Semantics Satisfaction A formula φ is • satisfiable in a model M if it is satisfied at some point of M • globally satisfied in M ( M | = φ ) if it is satisfied at all points in M • valid ( | = φ ) if it is globally satisfied in all models • a semantic consequence of a set of formulas Γ (Γ | = φ ) if for all models M and all points w , if M , w | = Γ then M , w | = φ 13 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Example: Hennessy-Milner logic Process logic (Hennessy-Milner logic) • PROP = ∅ • W = P is a set of states, typically process terms, in a labelled transition system • each subset K ⊆ Act of actions generates a modality corresponding to transitions labelled by an element of K → p ′ | K ⊆ Act }� as the K Assuming the underlying LTS F = � P , { p − modal frame, satisfaction is abbreviated as p | = � K � φ iff ∃ q ∈{ p ′ | p → p ′ ∧ a ∈ K } . q | = φ a − p | = [ K ] φ iff ∀ q ∈{ p ′ | p → p ′ ∧ a ∈ K } . q | = φ a − 14 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Example: Hennessy-Milner logic b S 2 S 3 Prove: a c 1 S 2 | = [ a ] ( � b � tt ∧ � c � tt ) c c S 1 2 S 1 �| = [ a ] ( � b � tt ∧ � c � tt ) c a 3 S 2 | = [ b ] [ c ] ( � a � tt ∨ � b � tt ) 4 S 1 | = [ b ] [ c ] ( � a � tt ∨ � b � tt ) S 4 S 5 b 15 / 52
� � What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Examples I An automaton a a � 2 b � 3 A = 1 b • two modalities � a � and � b � to explore the corresponding classes of transitions • note that 1 | = � a � · · · � a � � b � · · · � b � t where t is a proposition valid only at the (terminal) state 3. • all modal formulas of this form correspond to the strings accepted by the automaton, i.e. in language L = { a m b n | m , n > 0 } 16 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Examples II ( P , < ) a strict partial order with infimum 0 • P , x | = � false if x is a maximal element of P • P , 0 | = ♦ � false iff ... • P , 0 | = � ♦ � false iff ... 17 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Examples III Temporal logic • � T , < � where T is a set of time points (instants, execution states , ...) and < is the earlier than relation on T . • Thus, � ϕ (respectively, ♦ ϕ ) means that ϕ holds in all (respectively, some) time points. 18 / 52
What’s in a logic? Modal Logic Bisimulation and modal equivalence Richer modal logics Examples III � T , < � The structure of time is a strict partial order (i.e., a transitive and asymmetric relation) For any such structure, a new modality, � , can be defined based on the cover relation ⋖ for < ( i.e. , x ⋖ y if (1) every x < y and (2) there is no z such that x < z < y ). Thus, ∀ t ′ ∈{ p ′ | t ⋖ t ′ } . t ′ | t | = � φ iff = φ ∀ t ′ ∈{ p ′ | t < t ′ } . t ′ | t | = � φ iff = φ ∃ t ′ ∈{ p ′ | t < t ′ } . t ′ | t | = ♦ φ iff = φ 19 / 52
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