  Frege called the course-of-values of a concept \(F\) its extension. The extension of a concept \(F\) records just those objects which \(F\) maps to The True. Thus Basic Law V applies equally well to the extensions of concepts. Let ‘\(\phi(x)\)’ be an open sentence of any complexity with the free variable \(x\) (the variable \(x\) may have more than one occurrence in \(\phi(x)\), but for simplicity, assume it has only one occurrence). Then Frege would use the expression: