The argument in this difficult passage can accurately be summarized in standard form: Intuitively, one can think of the argument as being powered by two ideas.
The first, expressed by Premise 2, is that we have a coherent idea of a being that instantiates all of the perfections.
Accordingly, the very concept of a being that instantiates all the perfections implies that it exists. Gaunilo of Marmoutier, a monk and contemporary of Anselm's, is responsible for one of the most important criticisms of Anselm's argument.
It is quite reasonable to worry that Anselm's argument illegitimately moves from the existence of an idea to the existence of a thing that corresponds to the idea.
Thus, on this general line of argument, it is a necessary truth that such a being exists; and this being is the God of traditional Western theism.
This article explains and evaluates classic and contemporary versions of the ontological argument.
As the objection is sometimes put, Anselm simply defines things into existence-and this cannot be done.
Gaunilo shared this worry, believing that one could use Anselm's argument to show the existence of all kinds of non-existent things: Now if some one should tell me that there is …
There is, of course, this difference: whereas the concept of a bachelor explicitly contains the proposition that bachelors are unmarried, the concept of God does not explicitly contain any proposition asserting the existence of such a being.
Even so, the basic idea is the same: ontological arguments attempt to show that we can deduce God's existence from, so to speak, the very definition of God.