Anselm of Canterbury (1033-1109)

1. Anselm of Canterbury (1033-1109) Ontological Argument

2. Background 1. A priori 2.Valid Argument 3. Reductio ad Absurdum

3. A priori • A priori is an epistemological concept. • It says something about how we come to know a claim. • It is opposed to a posteriori.

4. A priori • A claim is a priori if and only if it is known through reason alone. • No experience is necessary for verifying the truth of the claim. • The truth of the claim can be determined simply by intellectual insight.

5. Examples of a priori claims • “2+2=4” • All definitions are a priori. • “A square has four sides.” • “A bachelor is an unmarried man.”

6. Valid • A valid argument: if the premises of the argument are true, then the conclusion follows necessarily. • A valid argument is an argument with perfect structure. • The structure of the argument is such that the conclusion follows necessarily from the premises. • In a valid argument it is logicallyimpossible for the premises to be true and the conclusion to be false. • So, if you accept the premises, then you MUST accept the conclusion.

7. ValidArguments: Examples Argument 1 1) If p then q 2) p 3) Therefore, q Argument 2 1) Either p or q 2) q is false 3) Therefore, p is true.

8. Reductio ad absurdum • This is a valid argument that uses the strategy of beginning by assuming the opposite of what you intend to show and deriving a contradiction.

9. Example of reductio ad absurdum Let us imagine you want to proof that “you are home” 1) If it is not the case that I am home (or if it is false that I am home), then my car would not be outside. 2) My car is outside. 3) Therefore, I am home.

10. Rationalism • Knowledge is derived from reason • Knowledge is apriori • Plato • Augustine • Anselm • Descartes • Spinoza • Leibniz

11. Empiricism • Knowledge is derived through the senses • Aristotle • St. Thomas • Aquinas • Locke • Berkeley • Hume