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The ontological argument is an a priori argument formulated
through reason alone, rather than experience. It is deductive as it
draws its argument purely from a set of premises. No claims of
truth are made, simply of logic.
In eleventh century, St. Anselm put forward his argument for the existence of God in his published
Proslogion. His first form (published in Proslogion 2) was based on four premises which led to the
conclusion that God must exist. His first premise was that, by definition, God is "that than which no
greater can be conceived". His second premise was that both believers and not believers accept this,
even `the fool' from Psalm 14. His third premise was that it is possible to exist either just in the mind,
or in the mind and in reality. The fourth premise was that it is better to exist both in the mind and in
reality. Therefore, Anselm concluded, God must exist both in the reality and in the mind.
Gaunilo the monk disagreed with this line of argument. He replied, on behalf of the fool, with "I can
conceive of a most perfect and real island" (premise one), then as Anselm's fourth premise it is
better to exist in the mind and in reality. This led the fool to the conclusion that this island must exist.
Gaunilo's third premise was that the notion of a real and perfect island is absurd so he said that by
association, so must be Anselm's argument.
Anselm's reply to this was published in `Proslogion 3' in which he stated that an island is contingent,
and that any contingent being can be conceived as not-existing. By contrast, Anselm claimed, a being
"than which no greater can be conceived" cannot not-exist. Therefore, he concluded, God's
existence is necessary.
Anselm also added that the notion of an island has no intrinsic maximum. It can always be bettered.
God cannot be bettered, so the two are not comparable.
In the 13th century Thomas Aquinas rejected this argument on the grounds that it has no basis as
there is no agreed definition of God.
Descartes, on the other hand, argued the opposite. He said that God's essence was his existence.:
I can conceive of a perfect being that has all the perfections.
I must assert all the perfections of this supremely perfect being.
Necessary existence is a perfection.
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Any being that has ever not-existed cannot be perfect.
Therefore a supremely perfect being, namely God, must exist.
Descartes claimed that as existence was the essence of God and a predicate of Him, it could be no
more separated from the idea of a God than `three angles' can be separated from a triangle.
In the 18th century Kant dismissed the Ontological argument. His two main arguments were that
existence is not a predicate, and that all existential propositions are synthetic.…read more
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Alvin Plantinga's line of argument centre's around the idea of `possible worlds' in `modal logic'.
Plantinga's line of argument went as follows:
1. The existence of a maximally excellent being in a possible world must be either necessary or
2. The existence of a maximally excellent being would only be impossible if it were logically
absurd, or self-contradictory.
3. It is neither, so it must be possible.
4. The concept of necessary and impossible cannot vary from world to world.