 Call a set of formulas \(\Gamma\) satisfiable if, and only if, there is a model \(\langle D, I\rangle\) and a variable assignment \(s\), which satisfies each formula \(A\) in \(\Gamma\) in the model. A set \(\Gamma\) of formulas is satisfiable if, and only if, every finite subset of \(\Gamma\) is satisfiable. (Compactness). This is a simple corollary of Completeness.