Existence is Not a Property
Summary
A bachelor can be defined as an unmarried man. Being unmarried is the essential defining property of a bachelor. Now, if I were to say ‘bachelors exist’, I would not be giving a further property to bachelors. Existence is not the same sort of thing as the property of being unmarried: for anyone to be unmarried they must first exist,
If we apply the same thinking to the Ontological Argument, we see that the mistake it makes is to treat the existence of God as if it were simply another property, like omniscience, or omnipotence. But God could not be omniscient or omnipotent without existing, so by giving a definition of God at all we are already assuming that he or she exists.
Listing existence as a further essential property of a perfect being is making the mistake of treating existence as a property rather than as the precondition of anything having any properties at all.
