The first aim of this paper is to try and determine what St. Anselm meant in his original argument in the Proslogion (ch. 2). This needs to be done because not only are the writers who expound his demonstration divided in their interpretations of it, and these interpretations quite different, but, very strangely, one does not find that they mention that there is any ambiguity and that other writers construe Anselm's words differently from themselves. Since there are in fact two arguments in the classical formulation of his proof, I want to try and disentangle them, decide which of them Anselm intendedand what is its connection with the demonstration in the third chapter of the Proslogion, and thus to offer an opinion on what exactly the ontological argument of St. Anselm is. Then on the basis of the clarification, I hope to show what it was that Gaunilo and St. Thomas attacked, to touch briefly on the relation of the 17th century ontological argument to Anselm's, and in general to indicate the curious round-aboutness of the historical criticism of Anselm. Anselm argued and Gaunilo and Thomas refuted something, though not, I think, precisely what Anselm meant, or, at any rate, not all that he meant. Actually it was Descartes that Thomas refuted by refuting half of Anselm. Kant refuted the other half of Anselm in an irrelevant criticism of Descartes. So that it could properly be said that Anselm's proof has been broken by philosophers who were either only partly aware of what he was maintaining, or who never had him in mind at all. Finally, because the important thing about the ontological argument, in my opinion, is not whether it is a demonstration of God's existence, but whether its refutation is a demonstration of the impossibility of God's existence, and because this question seems to arise most sharply in connexion with Anselm's position, I want to say something about that.