Kripke-Style Semantics for Kd

We define the frames for modeling Kd as follows:

F is an Kd Frame: F = <W, R, DEM> such that:

1. W is a non-empty set
2. R is a subset of W × W
3. DEM is a subset of W
4. ∀i∃j(Rij & j ∈ DEM).

A model can be defined in the usual way, allowing us to then define truth at a world in a model for all sentences of Kd (as well as for KTd):

M is an Kd Model: M = <F,V>, where F is an Kd Frame, <W,R,DEM>, and V is an assignment on F: V is a function from the propositional variables to various subsets of W.

Basic Truth-Conditions at a world, i, in a Model, M:

[PC]:	(Standard Clauses for the operators of Propositional Logic.)
[□]:	M ⊨i □p iff ∀j(if Rij then M ⊨j p).
[d]:	M ⊨i d iff i ∈ DEM.

Derivative Truth-Conditions:

[◊]:	M ⊨i ◊p: ∃j(Rij & M ⊨j p)
[OB]:	M ⊨i OBp: ∀j[if Rij & j ∈ DEM then M ⊨j p]
[PE]:	M ⊨i PEp: ∃j(Rij & j ∈ DEM & M ⊨j p)
[IM]:	M ⊨i IMp: ∀j[if Rij & j ∈ DEM then M ⊨j ~p]
[OM]:	M ⊨i OMp: ∃j(Rij & j ∈ DEM & M ⊨j ~p)
[OP]:	M ⊨i OPp: ∃j(Rij & j ∈ DEM & M ⊨j p) & ∃j(Rij & j ∈ DEM & M ⊨j ~p)

(Truth in a model and validity are defined just as for SDL.)

Metatheorem: Kd is sound and complete for the class of all Kd models.

Return to Deontic Logic.