K - Kripke Normal Modal Logic

K is the foundation of so-called normal modal logics. It adds modal operators and to CFOL.


Semantics

A K frame comprises the interpretation of sentences and predicates at a world. A K model comprises a non-empty collection of K frames, a world access relation \(R\), and a set of constants (the domain).

Truth Values

Common labels for the values include:

T

1

true

F

0

false

Truth Tables

The value of a sentence with a truth-functional operator is determined by the values of its operands according to the following tables.

Negation
¬
T F
F T
Conjunction
T F
T T F
F F F
Disjunction
T F
T T T
F T F

Defined Operators

The Material Conditional is definable in terms of disjunction:

A ⊃ B \(:=\) ¬A ∨ B

Likewise the Material Biconditional is defined in terms of and :

A ≡ B \(:=\) (A ⊃ B) ∧ (B ⊃ A)
Material Conditional
T F
T T F
F T T
Material Biconditional
T F
T T F
F F T

Compatibility Tables

K does not have separate Assertion or Conditional operators, but we include tables and rules for them, for cross-compatibility.

Assertion
T T
F F
Conditional
T F
T T F
F T T
Biconditional
T F
T T F
F F T

Predication

The value of predicated sentences are handled in terms of a predicate's extension.

A sentence with n-ary predicate \(P\) over parameters \(\langle a_0, ... ,a_n\rangle\) has the value T iff \(\langle a_0, ... ,a_n\rangle\) is in the extension of \(P\).

Quantification

Existential

An existential sentence is true just when the sentence resulting in the subsitution of some constant in the domain for the variable is true.

Universal

A universal sentence is true just when the sentence resulting in the subsitution of each constant in the domain for the variable is true.

Consequence

Logical Consequence is defined similary as CPL, except with reference to a world:

Logical Consequence

C is a Logical Consequence of A iff, all models where the value of A is T at \(w_0\) are models where C also has the value T at \(w_0\).

Tableaux

Nodes

Nodes for bivalent modal tableaux come in two types:

  • A sentence-world pair like \(A, w\), indicating that \(A\) is true at \(w\).

  • An access node like \(w_1Rw_2\), indicating that the pair \(\langle\ w_1, w_2\ \rangle\) is in the access relation \(R\).

Trunk

To build the trunk for an argument, add a node with each premise, with world \(w_0\), followed by a node with the negation of the conclusion with world \(w_0\).

To build the trunk for the argument A1 ... AnB write:
A1, w0
An, w0
¬B, w0

Closure

Contradiction Closure[source]
A, w0
¬A, w0
¬=Self Identity Closure[source]
a ≠ a, w0
¬E!Non Existence Closure[source]
¬E!a, w0

Rules

In general, rules for connectives consist of two rules per connective: a plain rule, and a negated rule. The special case of negation only one rule for double negation.

There is also a special rule for the Identity predicate.

Additional rules are given for the quantifiers.

Operator Rules

¬ Rules
¬¬Double Negation[source]
¬¬A, w0
A, w0
∧ Rules
Conjunction[source]
A ∧ B, w0
A, w0
B, w0
¬Conjunction Negated[source]
¬(A ∧ B), w0
¬A, w0
¬B, w0
∨ Rules
Disjunction[source]
A ∨ B, w0
A, w0
B, w0
¬Disjunction Negated[source]
¬(A ∨ B), w0
¬A, w0
¬B, w0
⊃ Rules
Material Conditional[source]
A ⊃ B, w0
¬A, w0
B, w0
¬Material Conditional Negated[source]
¬(A ⊃ B), w0
A, w0
¬B, w0
≡ Rules
Material Biconditional[source]
A ≡ B, w0
¬A, w0
¬B, w0
B, w0
A, w0
¬Material Biconditional Negated[source]
¬(A ≡ B), w0
A, w0
¬B, w0
¬A, w0
B, w0

Predicate Rules

= Rules
=Identity Indiscernability[source]
Fa, w0
a = b, w0
Fb, w0

Quantifier Rules

∃ Rules
Existential[source]
∃xFx, w0
Fa, w0
¬Existential Negated[source]
¬∃xFx, w0
∀x¬Fx, w0
∀ Rules
Universal[source]
∀xFx, w0
Fa, w0
¬Universal Negated[source]
¬∀xFx, w0
∃x¬Fx, w0

Compatibility Rules

⚬ Rules
Assertion[source]
⚬A, w0
A, w0
¬Assertion Negated[source]
¬⚬A, w0
¬A, w0
→ Rules
Conditional[source]
A → B, w0
¬A, w0
B, w0
¬Conditional Negated[source]
¬(A → B), w0
A, w0
¬B, w0
↔ Rules
Biconditional[source]
A ↔ B, w0
¬A, w0
¬B, w0
B, w0
A, w0
¬Biconditional Negated[source]
¬(A ↔ B), w0
A, w0
¬B, w0
¬A, w0
B, w0

Notes

References

Futher Reading