 Saturation Principle. Let \(\Phi\) be a collection of \(\cL(U)\)-formulas with exactly one free variable. If \(\Phi\) is finitely satisfiable in \(\fU\), that is, if for any finite subset \(\Phi '\) of \(\Phi\) there is an element of \(U\) which satisfies all the formulas of \(\Phi '\) in \(\fU\), then there is an element of \(^*U\) which satisfies all the formulas of \(\Phi\) in \(^*\fU\).