PHI 101 Chapter Notes - Chapter 8: Ontological Argument, Gaunilo Of Marmoutiers
13 Jan 2017
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Chapter 8: The Ontological Argument
1. A posteriori and A Priori
a. A posteriori means that a truth requires experience to be known or justified
b. A priori propositions can be known to be true through reason alone
i. Mathematical truths and definitions
2. Definitions and existence
a. A definition doesn’t imply that the things you are defining exist
i. They just specify what it would take to be an individual of the kind in
question
b. The ontological argument claims that the concept of God is different: from the
definition of the concept of God, the existence of God is supposed to follow
3. Anselm’s Argument
a. (1) God is by definition the greatest being possible.
b. (2) A being who fails to exist in the actual world (while existing in other possible
worlds) is less perfect than a being who exists in all possible worlds)
c. © Hence, God exists,necessarily
d. Premise One: Conceivability and Possibility
i. Intended to capture what Anselm meant by saying that God is a being
“than which none greater can be conceived”
1. God is the best possible being
ii. What is possible is an objective question: it doesn’t depend on what
people know or believe
iii. Whether we can conceive of something, however, is a fact about us as
knowing subjects
1. Conceivability is a subjective notion
2. Depends on individual
iv. God is not supposed to be the best being people can conceive. Rather, he
is the best possible being.
e. Premise 2: Necessary Existence is a perfection
i. A truly perfect being does not depend on anything for its existence
ii. Necessary beings are more perfect than contingent beings so god needs to
be necessary
4. Gaunilo’s Criticism
a. The argument must be defective, since if it were not, you could prove the
existence of a perfect island by similar a priori argument
b. Objection to his criticism
i. Concept of islandhood resembles that of bachelorhood
1. Definition doesn’t deductively imply that there are any objects of
the kind defined
2. Any attempt to provide an a priori proof that islands exist must fail
c. Let’s define the concept of a p-island, which is by the definition the best possible
island
i. Gaunilo thought that there is no a priori demonstration that there are p-
islands
1. Even if p-islands were real, they can’t be proved by a priori means
d. Hence, Anselm’s argument must have a mistake as well
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