In the history of human thought, various ways have been proposed for proving the existence of God. Among the proposed arguments, the most controversial one is the ontological argument. The ontological argument tries to prove the existence of God through analyzing the concept of God. This argument goes beyond any empirical concept. Historically speaking, this argument dates back to Aristotle, but, in western philosophy, it was Anselm who introduced it as an argument for the first time in his Proslogion. Since Anselm’s time, pros and cons have written in detail to prove their position. Among them, Descartes’ defence of this argument in the 17th century has attracted the attention of thinker in a special way. The different views expressed by some of the contemporary Moslem philosophers on the validity or lack of validity of this argument are also interesting. Altogether, with close attention, the ontological argument provides an invalid ambiguous reason which is based on a fallacy confusing the concept of existence with its denotation.