  In [1944] Hailperin gave the first of a number of finite axiomatisations of NF now known. Many of them exploit the function \(x \mapsto \{y : x \in y\}\) which is injective and total and is an \(\in\)-isomorphism. This function was known to Whitehead, who suggested to Quine that \(\{y : x \in y\}\) should be called the “essence” of \(x\) (a terminology clearly suggested by a view of sets as properties-in-extension).