Consider this argument: One can conceive of a maximally perfect being. If one can conceive of such a being, then such a being does not merely exist as a conception. This is true because if it existed only as a conception (if it only existed in one's mind) then one could think of an even greater being that had all the attributes of the first but also existed in real life (since it is clearly greater to exist in real life than to exist merely as a conception). But in thinking of this second being we come to a contradiction since there can't be a being greater than the greatest possible being. Therefore, the maximally perfect being that exists in real life is the being we were originally thinking of. Therefore, a maximally perfect being exists. when you think of the first supposedly perfect being, it is already in question whether that being exists or not. it is not included in its specification that it does not exist. so thinking of another being like it, only it exists, isn't more perfect, unless perfection isn't only in the idea of something but also in its ontological existence - in which case, whether we really are thinking of a more perfect being is predicated on his external existence, which we still don't necessarily know. the moral of this story: if it looks like a sophist and acts like a sophist, it's probably a sophist. now i've come across a different formulation of the argument which requires a different refutation. "God, by definition, is that than which none is greater. God exists in the understanding. If God exists in the understanding, we could imagine Him to be greater by existing in reality. Therefore, God must exist." to imagine or understand god is not a priori to imagine that he exists in understanding, so imagining that he exists in reality, rather than just in understanding, is not opposed to his existing only in understanding, it is opposed to imagining that he only exists in the understanding, which is something different, and a straw man. if you imagine he exists in reality because he is greater that way, you still don't know if your imagination that he exists in reality matches the reality outside of your imagination, which means that you don't know if god is greater than existing in the imagination, hence you don't know if god exists if god is defined as existing outside of the imagination (a la being greater), and if god isn't defined as existing outside of the imagination, then there's no reason to be compelled to imagine that he exists just by virtue of imagining him. i did contradict myself when i implied that god can even exist in the imagination, but so did the proof when it stated that god exists in the imagination, and then implied that god is defined as not existing in the imagination because otherwise he wouldn't meet the criteria of being greater. basically, imagining he exists doesn't make him exist, and the reason we imagine it doesn't matter. if we imagine it because we want a greater god, we either don't necessarily know that the greater god exists, or we can't imagine god in the first place unless he does exist. the trick of this proof is to say that god that which there is none greater than and then to posit a god that is less than as valid in order to validate the god that is greater than it. but if god is only the existent god then he is not the same god you imagined. as far as we're able to compare greatness, we're still imagining, and as far as we're still imagining, we still can't tell if what we're imagining exists in this proof, to imagine that god exists in reality is to imagine that we are imagining that he exists in reality, whereas his really existing in reality would be synonymous with our imagining he exists in reality and not with our imagining that we are imagining he exists in reality. Kant said that existence is not a predicate. The problem is that existence is a predicate, but it relates to the relationship between the idea and the experience. This proof of god takes experience out of the loop. What does it mean that God 'exists in reality' if the reasons that we determined he exists have no implications about what we will perceive or experience? This gets to the fact that 'greater than' is way too vague a notion to be used in pure logic. If you want to define greater than as existing in reality rather than just imagination, fine, but that doesn't prove anything about experience. If you want to define greater than as implying other things, such as being the Most Intelligent, then proving he exists from scratch and then saying you will witness this intelligence is like saying that .. well I can't think of an example, but anyway, 'greater' is way too vague a term to base a proof on. It's just a way of defining God as a) existent b) the most intelligent c) the most beautiful d) the biggest etc. etc. etc. and then saying that since a is true, all the rest are true.