REASONING ABOUT GOD
Anselm’s first form of the argument is like this:
P1. God is the greatest possible being (nothing greater can be conceived)
P2 God exists only in the imagination
P3. If God exists in the mind alone, then a greater being could exist in the mind AND in reality
C1. This being would then be greater than God
P4. Either P1 or P2 must be false, but P1 cannot be false, so P2 must be false instead.
C2. Therefore, God exists both in the mind AND in reality.
The Ontological Argument "unpacks" the definition of God. The FACT of God's existence is contained in God's ESSENCE. Imagining a non-existent God is as contradictory and illogical as imagining a triangle without 3 sides (an analogy Descartes went on to make)..
The argument is based on agreed premises. The definition of God as "the most perfect being imaginable" or "that than which nothing greater can be conceived" ties in with religious experiences and with the presentation of God in the Bible (eg Job 38-41). This argument produces deductive certainty, which is the faith that moves mountains (Matthew 17: 20).
It's one thing to imagine God, but another thing to prove he exists. You can't move from definitions to reality. If the Ontological Argument looks like it proves something about the real world, then it must be making a logical mistake or confusing the definition of God, existence or perfection.
Faith should be based on evidence, not definitions. David Hume said, "A wise man proportions his belief to the evidence." A persuasive deductive argument should be based on what we observe the world to be like, not on the meaning of words. Richard Dawkins calls it "logomachist trickery" - 'logomachist' means arguing for the sake of arguing.