My aim in this paper is to critically assess Plantinga’s modal ontological argument for existence of God, such as it is presented in the book “The Nature of Necessity”. Plantinga tries to show that this argument is valid and it is rational to believe in his main premise, namely “there is a possible world in which maximal greatness is instantiated”. On the one hand, I want to show that this argument is logically valid in both systems B and S5 of (...) modal logic. On the other hand, I think that this argument is not a good argument to show that God exists or that it is rational to believe in God. (shrink)