1. Challenges to the OAs The different versions of OA are challenged by:
Gaunilo
Thomas Aquinas
Immanuel Kant
David Hume
Bertrand Russell
Anthony Kenny

2. General criticisms of Anselm’s OA
Anselm is trying to explain God’s existence from the premise that God exists.
All the OA does is place God’s existence in philosophically rational terms, if he DID exist.

3. General criticisms of Anselm’s OA
Can one conceive of God, or the Greatest Possible Being?
Does the GPB contain everything, including evil?
This is a circular argument which relies upon prior belief and does not include the possibility that God does not exist.

4. Thomas Aquinas Like Anselm, Aquinas believed in God.
His approach was Aristotelian, whereas Anselm was a Platonist.
He believed that we have to arrive at a truth (including arguing about God’s existence) by starting with an observation (empiricism).
Key texts: Summa Theologica and Summa Contra Gentiles.

5. Aquinas’ criticisms
An a priori argument that seeks to prove the existence of God using the nature or definition of His being.
God’s ontology/ essence/ nature and existence are not self-evident (i.e. true by definition).
“…because we do not know the essence of God…[this] needs to be demonstrated by things that are more known to us – namely, by effects.” {ST}

6. Aquinas’ criticisms
Mere humans cannot understand God’s essence as our souls are trapped in material matter.
“….the divine nature cannot be known through material things…” {ST}

7. Aquinas’ criticisms
It is a reductio ad absurdum argument which means it aims to show that a proposition is true because its denial entails a contradiction or some other absurdity!
Aquinas rejects the claim that it is contradictory to deny the existence of God.
“…because of a failure to distinguish between that which is self-evident, in an absolute sense and that which is self-evident in relation to us.” {ST}

8. Aquinas’ criticisms
It is analytic – which means it is impossible to think it is false (e.g. triangles have 3 sides).
Even if we accept the idea of TTWNGCBC it does not follow that it must exist.
Furthermore the concept of TTWNGCBC is precisely “not admitted by those who hold that God does not exist.” {ST}

9. CONCLUSION
Aquinas is not criticising Anselm for arguing that God exists, but how he goes about it.
We cannot know God’s nature so cannot claim part of his essence is existence.
It is logically possible to think of God not existing.
It is possible to think of something which is the GREATEST but for that only to exist in the mind.

10. CONCLUSION
God’s nature can only be understood via using our reason from observing a posteriori empirical effects to cause, plus Special Revelation.
For Aquinas we can only know God exists by looking at the world around us, which is his handiwork; through Scripture; and through supernatural experiences.

11. Homework - Due Monday 23rd Jan.
What is Anselm’s definition of God? (2)
Explain Gaunilo’s criticism of Anselm’s OA. (5)
It is illogical to think of the concept of God without the concept of existence. (15)