By defining God as a maximally great being Plantinga is able to devise an ontological argument which validly infers from the possibility of there being a God that there necessarily is a God. In this article I shall argue that Plantinga’s argument is not only question-begging, as several critics have complained, but circular in the strongest sense of the term. Based on reflections on the relation between the notions of coherence and possibility, I shall defend two arguments, previously proposed by (...) Tooley and Guleserian, against the existence of a maximally great being. The article concludes with some critical remarks on Plantinga’s rationale for conceiving God as such a being. (shrink)