It will be shown that propositional tense logic (with the Kripke relational semantics) may be regarded as a fragment of propositional modal logic (again with the Kripke semantics). This paper deals only with model theory. The interpretation of formal systems of tense logic as formal systems of modal logic will be discussed in [6].

The languages M and T, of modal and tense logic respectively, each have a countable infinity of propositional variables and the Boolean connectives; in addition, M has the unary operator ⋄ and T has the unary operators F and P. A structure is a pair <W, ≺<, where W is a nonempty set and ≺ is a binary relation on W. An assignment V assigns to each propositional variable p a subset V(p) of W. Then V(α)£ W is defined for all formulas α of M or T by induction:

We say that α is valid in <W, ≺>, or <W, ≺>, ⊨ α, if ⊩ (α) = W for every assignment V for <W, ≺>. If Γ is a set of formulas of M [T] and α is a formula of M [T], then α is a logical consequence of Γ, or Γ ⊩ α,if α is valid in every model of Γ i.e., in every structure in which all γ ∈ Γare valid.