PHIL1003 Lecture Notes - Lecture 5: Reductio Ad Absurdum, Ontological Argument
Anselm’s Ontological Argument
• What is an ontological argument?
o Attempts to prove the existence of god through the nature or
definition of God.
o Rather than appealing to the nature of the world, it uses a priori
evidence
• Existence
o Existence in understanding vs. existence in reality
▪ We need to have some understanding of the thing whose
existence is being denied before we can meaningfully deny
its existence
▪ When we asserts that something exists we are saying that it
exists both in understanding and reality
o Anyone who claims that God does not exist is committed, at least,
to asserting that God exists in the understanding
o Anselm argued that anyone who concedes God exists in
understanding also concedes that God exists in reality
• A being than which none greater is conceived
o He sought to prove that not only the greatest existing being exists
but the greatest conceivable being exists
o Therefore, there is no conceivable being greater than God
o If God is the greatest conceivable being then he must exist – or else
he would not be the greatest conceivable being.
▪ God exists through the mere existence of the definitions of a
God
o For any specific x, if x exists in reality as in the understanding, and
y exists in understanding only, then x is greater than y
• Greatness
o Perhaps there are two definitions of greatness
▪ 1 – worthy qualities of positive value; knowledge,
benevolence, power
▪ 2 – ontological perfection; existing in reality is greater than
not existing in reality
o The most perfect conceivable being must be great 1 + great 2.
• Reductio Ad absurbdum
o A technique of arguing whereby you begin by supposing that the
conclusion you want to argue for is false
o You then, with the help of other plausible statements (premises),
derive a contradiction (or falsehood) from that supposition.
o If the supposition leads to a contradiction, then it cant possibly be
true; the supposition must be false
o Hence you establish that the conclusion that you originally wished
to argue for is true.
• ^ In Anselms argument
o P1 - God exists in the understanding alone
o P2 – Existence in reality is greater than existence in understanding
alone
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