 In some classic essays (1987 and 1986), Boolos appears to recommend this very procedure of using separate existence and identity principles. In those essays, he eschews the primitive mathematical relation of set membership and suggests that Frege could formulate his theory of numbers (‘Frege Arithmetic’) by using a single nonlogical comprehension axiom which employs a special instantiation relation that holds between a concept \(G\) and an object \(x\) whenever, intuitively, \(x\) is an extension consisting solely of concepts and \(G\) is a concept ‘in’ \(x\). He calls this nonlogical axiom ‘Numbers’ and uses the notation ‘\(Gηx\)’ to signify that \(G\) is in \(x\):