Definition of God
St.Thomas Aquinas argued we can't prove that God exists just from the concept or idea of God.
We have no way of knowing if our definition of God is correct.
The definition conveys meaning - whatever you believe about God, its reasonable to say that nothing can be thought to be greater than God.
Anselm was presenting his argument/definition as a prayer to aid faith rather than as a proof of God.
Recent forms of the argument claim that an argument based on a definition is valid.
Deriving existential claims from definition
The argument implies you can define anything into existence - which is false.
Definitions only tell us what God would be like IF he existed. They can't establish whether he does actually exist. e.g/ describing a unicorn does not make the unicorn exist.
Anselm's response was that Islands are contingent and therefore don't have necessary existence as an aspect of their properties. But God does, He is a different order of being and is unique in this aspect.
Existence is not part of the greatness of an island; necessary existence is part of the concept of God.
Explaining concepts can make non-existence apparent. e.g/ round squares can't exist. Therefore concept and actuality are related.
Existence as a predicate of God
Immanuel Kant argued existence is not a real predicate. It doesn't add to our concept of the subject.
To say a concept existed is to state that it had an actuality. e.g/ to say that a kangaroo exists doesn't add to our concept of a kangaroo - existence isn't something that can be added or subtracted from something.
Necessary existence is a property of an inability to be generated or made corrupt.
Stephen Davis argued the mere concept of money doesn't have the property of enabling him to purchase anything in the real world. Therefore existence does add a 'great making quality' and permits the purchase of real items in the real world.