According to McKinsey and Tarski [3], who introduced the notion, a closure algebra Γ = (K, ∪, ∩, –, C) is a Boolean algebra (K, ∪, ∩, –) with an additional unary operator C, closed in K, such that, for every x, y ϵ K, x ⊆ Cx, CCx = Cx, C(x∪y) = Cx∪Cy, CΛ = Λ. McKinsey [2] has proved that a matrix Γ = (K, D, ∪, ∩, –, C) is a representation (normal matrix) of S4, if and only if (1) (K, ∪, ∩, –, C) is a closure algebra, (2) D is an additive ideal of K, (3) if – Cx ϵ D then x = Λ, D being the distinguished proper subset of K and ∪, ∩, –, C having been made to correspond to ∨, ·, ∼, ◊, respectively. Following McKinsey, one proves immediately

Theorem 1. A matrix Γ is a representation of S5, if and only if it is a closure algebra and

(α) for every x ϵ K, –C–Cx = Cx, and

(β) D is an additive ideal of K such that, for every x ϵ D, Cx = V.

This raises the question of alternative characterizations of S5. Because of (β), one may without substantial loss of generality put D = {V}. The question then becomes one for conditions that are in closure algebras equivalent to (α). One such characterization has been given by McKinsey and Tarski [4]. Defining x∸y = C(x∩–y) and ¬x = V∸x, these authors have shown that the closed elements K* of a closure algebra Γ form a Brouwerian algebra Γ* = (K*, ∪, ∩, ∸), and furthermore that Γ* is a Boolean algebra under ∪, ∩, ¬, if and only if, for every x ϵ K*,