existential instantiation a rule of inference admissible in classical quantification theory. It allows one to infer a statement A from an existentially quantified statement DxB if A can be inferred from an instance B(a/x) of DxB, provided that a does not occur in either A or B or any other premise of the argument (if there are any). (Intuitively, it allows one to infer a contradiction C from ‘There exists a highest prime’ if C can be inferred from ‘a is a highest prime’ and a does not occur in C.) Free logic allows for a stronger form of this rule: with the same provisions as above, A can be inferred from DxB if it can be inferred from B(a/x) & E!a. (Intuitively, it is enough to infer ‘There is a highest natural number’ from ‘a is a highest prime and a exists’.) See also FORMAL LOGIC , FREE LOGI. E.Ben.