 The idea of minimization is at the basis of one of the earliest non-monotonic formalisms, McCarthy’s circumscription  (McCarthy 1980).  Circumscription makes explicit the intuition that, all other things being equal, extensions of certain predicates should be minimal. Consider principles such as “all normal birds fly”. Implicit in this principle is the idea that specimens should not be considered to be abnormal unless there is positive information to that effect. McCarthy’s idea was to represent this formally, using second-order logic (SOL). In SOL, in contrast to first-order logic (FOL), one is allowed to explicitly quantify over predicates, forming sentences such as \(\exists P \forall x Px\) (“there is a universal predicate”) or \(\forall P (Pa \equiv Pb)\) (“a and b are indiscernible”). 