Not every frame property definable by modal formulas is first- order definable. However, if we allow quantification over one- place predicates (i.e., monadic second-order quantification), we define all modally definable frame properties. The trick is to exploit a systematic way in which the conditions under which a modal formula is true at a world are related to first-order formu- las. This is the so-called standard translation of modal formulas into first-order formulas in a language containing not just a two- place predicate symbol Q for the accessibility relation, but also a one-place predicate symbol Pi for the propositional variables pi
occurring in A.
Definition 2.15. The standard translation STx(A) is inductively
defined as follows:
1. A ≡ ⊥: STx(A) = ⊥.
35 2.7. SECOND-ORDER DEFINABILITY 3. A ≡ ¬B: STx(A) = ¬STx(B). 4. A ≡ (B ∧ C ): STx(A) = (STx(B) ∧ STx(C )). 5. A ≡ (B ∨ C ): STx(A) = (STx(B) ∨ STx(C )). 6. A ≡ (B → C ): STx(A) = (STx(B) → STx(C )). 7. A ≡ □B: STx(A) = ∀y (Q (x, y) → STy(B)). 8. A ≡ ♢B: STx(A) = ∃y (Q (x, y) ∧ STy(B)).
For instance, STx(□p → p) is ∀y (Q (x, y) → P (y)) → P (x). Any structure for the language of STx(A) requires a domain, a two-
place relation assigned to Q , and subsets of the domain assigned to the one-place predicate symbols Pi. In other words, the com-
ponents of such a structure are exactly those of a model for A: the domain is the set of worlds, the two-place relation assigned to Q is the accessibility relation, and the subsets assigned to Pi are
just the assignments V (pi). It won’t surprise that satisfaction of
A in a modal model and of STx(A) in the corresponding structure
agree:
Proposition 2.16. Let M = ⟨W, R,V ⟩, M′ be the first-order struc- ture with |M′| = W , QM ′ = R, and PM′ i = V (pi), and s (x) = w. Then M, w ⊩ A iff M′, s ⊨ STx(A) Proof. By induction on A. □
Proposition 2.17. Suppose A is a modal formula and F = ⟨W, R⟩ is a frame. Let F′be the first-order structure with |F′| = W and QF
′
= R, and let A′be the second-order formula
36 CHAPTER 2. FRAME DEFINABILITY
where P1, . . . , Pn are all one-place predicate symbols in STx(A). Then
F ⊨ A iff F′⊨ A′
Proof. F′ ⊨ A′ iff for every structure M′ where PM
′
i ⊆ W for
i = 1, . . . , n, and for every s with s (x ) ∈ W , M′, s ⊨ STx(A). By Proposition 2.16, that is the case iff for all models M based on F and every world w ∈ W , M,w ⊩ A, i.e., F ⊨ A. □
Definition 2.18. A class C of frames is second-order definable if there is a sentence A in the second-order language with a single two-place predicate symbol P and quantifiers only over monadic set variables such that F = ⟨W,R⟩ ∈ C iff M ⊨ A in the struc- ture M with |M|= W and PM = R.
Corollary 2.19. If a class of frames is definable by a formula A, the corresponding class of accessibility relations is definable by a monadic second-order sentence.
Proof. The monadic second-order sentence A′ of the preceding proof has the required property. □ As an example, consider again the formula □p → p. It de- fines reflexivity. Reflexivity is of course first-order definable by the sentence ∀x Q (x, x). But it is also definable by the monadic second-order sentence
∀X ∀x (∀y (Q (x, y) → X (y)) → X (x)).
This means, of course, that the two sentences are equivalent. Here’s how you might convince yourself of this directly: First suppose the second-order sentence is true in a structure M. Since x and X are universally quantified, the remainder must hold for any x ∈ W and set X ⊆ W , e.g., the set {z : Rxz } where R = QM. So, for any s with s (x) ∈ W and s (X ) = {z : Rxz } we have
37 2.7. SECOND-ORDER DEFINABILITY M ⊨ ∀y (Q (x, y) → X (y)) → X (x). But by the way we’ve picked
s (X ) that means M, s ⊨ ∀y (Q (x, y ) → Q (x, y )) → Q (x, x ), which is equivalent to Q (x, x) since the antecedent is valid. Since s (x) is arbitrary, we have M ⊨ ∀x Q (x, x).
Now suppose that M ⊨ ∀x Q (x, x) and show that M ⊨ ∀X ∀x (∀y (Q (x, y) → X (y)) → X (x)). Pick any assignment s , and assume M, s ⊨ ∀y (Q (x, y) → X (y)). Let s′ be the y-variant of s with s′(y) = x; we have M, s′ ⊨ Q (x, y) → X (y), i.e., M, s ⊨ Q (x, x ) → X (x ). Since M ⊨ ∀x Q (x, x ), the antecedent is true, and we have M, s ⊨ X (x), which is what we needed to show.
Since some definable classes of frames are not first-order de- finable, not every monadic second-order sentence of the form A′ is equivalent to a first-order sentence. There is no effective method to decide which ones are.
Problems
Problem 2.1. Complete the proof ofTheorem 2.1.
Problem 2.2. Prove the claims inProposition 2.2.
Problem 2.3. Let M = ⟨W,R,V ⟩ be a model. Show that if R satisfies the left-hand properties of table 2.2, every instance of the corresponding right-hand formula is true in M.
Problem 2.4. Show that if the formula on the right side of ta- ble 2.2is valid in a frame F, then F has the property on the left side. To do this, consider a frame that does not satisfy the prop- erty on the left, and define a suitable V such that the formula on the right is false at some world.
Problem 2.5. ProveProposition 2.9.
Problem 2.6. ProveProposition 2.12by showing:
38 CHAPTER 2. FRAME DEFINABILITY
2. If R is reflexive, it is serial.
3. If R is reflexive and euclidean, it is symmetric. 4. If R is symmetric and euclidean, it is transitive. 5. If R is serial, symmetric, and transitive, it is reflexive. Explain why this suffices for the proof that the conditions are equivalent.
CHAPTER 3
Axiomatic
Derivations
3.1
Introduction
We have a semantics for the basic modal language in terms of modal models, and a notion of a formula being valid—true at all worlds in all models—or valid with respect to some class of models or frames—true at all worlds in all models in the class, or based on the frame. Logic usually connects such semantic charac- terizations of validity with a proof-theoretic notion of derivability. The aim is to define a notion of derivability in some system such that a formula is derivable iff it is valid.
The simplest and historically oldest derivation systems are so-called Hilbert-type or axiomatic derivation systems. Hilbert- type derivation systems for many modal logics are relatively easy to construct: they are simple as objects of metatheoretical study (e.g., to prove soundness and completeness). However, they are much harder to use to prove formulas in than, say, natural deduc- tion systems.
In Hilbert-type derivation systems, a derivation of a formula is a sequence of formulas leading from certain axioms, via a handful of inference rules, to the formula in question. Since we want the derivation system to match the semantics, we have to guarantee
40 CHAPTER 3. AXIOMATIC DERIVATIONS
that the set of derivable formulas are true in all models (or true in all models in which all axioms are true). We’ll first isolate some properties of modal logics that are necessary for this to work: the “normal” modal logics. For normal modal logics, there are only two inference rules that need to be assumed: modus ponens and necessitation. As axioms we take all (substitution instances) of tautologies, and, depending on the modal logic we deal with, a number of modal axioms. Even if we are just interested in the class of all models, we must also count all substitution instances of K and Dual as axioms. This alone generates the minimal nor- mal modal logic K.
Definition 3.1. The rule of modus ponens is the inference schema A A → B mp
B
We say a formula B follows from formulas A, C by modus ponens iff C ≡ A → B.
Definition 3.2. The rule of necessitation is the inference schema A nec
□A
We say the formula B follows from the formulas A by necessitation iff B ≡ □A.
Definition 3.3. A derivation from a set of axioms Σ is a sequence of formulas B1, B2, . . . , Bn, where each Bi is either
1. a substitution instance of a tautology, or
2. a substitution instance of a formula in Σ , or