Being an eminent mathematician and philosopher, Leibniz suggests that necessary judgements are necessary truths, identity judgments and these are the source of ideal knowledge. But Leibnizian concept of necessity is ceased to exist if the theory of A.C. Ewing and the principle of uncertainty in Quantum mechanics of the Great German Scientist Werner Heisenberg are accepted. This paper is a critical appraisal of Leibinizian concept of necessity.