Def: A well-formed formula f (of sentential logic) is *satisfiable* if there exists a truth assignment t such that f is true under t. Def: A set S of well formed formulaes is *satisfiable* if there is a truth assignment t that satisfies every formula of S. Theorem [Compactness Theorem] ----------------------------- Let S be a set of well-formed formulaes. Suppose that every finite subset of S is satisfiable. Then S is satisfiable.