1. Aquinas’ Proofs
The five ways

2. Thomas Aquinas ( )
Joined Dominican order against the wishes of his family; led peripatetic existence thereafter.
Considered the most learned man of his day; much in demand as teacher and lecturer.
Summa Theologica never finished, following ‘ecstasy’ in Dec. 1273
Doctor of the Church

3. Aquinas on God’s existence
Believed, as against several interesting objections, that God’s existence can (and needs to be) ‘demonstrated’ (‘proved’, in the modern sense).
By this he meant 2 things:
1. That God exists is not ‘self-evident’ or axiomatic or a matter of definition
2. That God exists is something which we can be completely sure of, as a matter of reason (and not simply of faith)

4. Is the existence of God self-evident?
“self-evident” = that which requires no proof in order to be known.
3 objections: the existence of God is self-evident because:
I. Knowledge of God is innate
II. The concept includes existence
III. God is truth, & no one can consistently deny the existence of truth.

5. Answers to the objections
Objection 1: we know only in a vague way (“God is man’s beatitude”) that God exists, and this is quite different from knowing absolutely that He exists.
Obj.2: distinction between mental and real existence maintained even in the case of “that than which no greater can be thought”
Obj.3: “The existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us.”
General rejoinder: what is self-evident cannot be denied, but “God is” can be denied.
Therefore, “God is” is not self-evident (that is, his existence requires proof)

6. Objections to the idea that God’s existence can be proven
Objection 1: God’s existence is a matter of faith, not demonstration (reason)
Obj. 2: God, by definition, exceeds our understanding, therefore we can’t even know what it is we’re trying to prove the existence of (“the essence is the middle term of demonstration”)
Obj.3: We can’t know God directly, only by his effects; but His effects (as finite objects or events) can’t tell us anything about His nature (which is, by dfn., infinite). Therefore, we can’t prove anything about Him.

7. Replies to the objections
Objection 1: Anything which can be known by “natural reason” is not an article of faith -- something is an article of faith only if it cannot be known by other means
Obj.2: We don’t need to know what it is that we’re proving the existence of (that is, it’s essence), only that it exists
Obj.3: As in 2, we’re only trying to prove that He exists and not anything about His nature. Every effect indicates, at the very least, the existence of its cause.
General rejoinder: 2 ways of proving things: a priori and a posteriori; God’s existence can only be proven in the latter way

8. Character of Aquinas’ proofs
Aquinas distinguishes a priori (from cause to effect; from the nature of something to its consequences) from a posteriori (from effect to cause; from observable consequences to the nature of what causes those) proofs -- the first are deductive proofs, what are the second kind? Are they proofs at all? (recall our previous discussion of “proof”?)

9. Proof 1: argument from motion
“motion” Aquinas understands to be a paradigm case of change; the argument here is better thought of as the argument from change
Change = going from potential to actual
Every such ‘move’ requires something which is itself actual to begin with
A chain of such ‘moves’ cannot be infinitely long
Therefore, there must be a ‘first mover’ (a first initiator of change), which is not itself moved; this first mover is God.

10. Proof 2: argument from efficient cause
Aquinas, following Aristotle, recognizes four kinds of ‘causes’ (4 kinds of ‘why’): formal, material, final, and efficient causes
Nothing is the efficient cause of itself; therefore, for every effect there must exist some efficient cause distinct from the effect
Such a chain of causes cannot go on to infinity; therefore there is a first cause (and that is God)

11. Proof 3: argument from possibility
Everything which exists, exists only contingently (that is, it is possible that it could not exist)
Any contingent being must have, at some time, not existed (if it is possible that it not exist at this time, then necessarily it did not exist at some time)
The world, taken as a whole & as contingent, must have at some time therefore not existed
But what once did not exist must come to exist in virtue of something which itself must exist.
That necessary being is God.

12. Proof 4: argument from gradation
Every quality or attribute which can be thought of in terms of degree, is referenced to some standard (more or less good to some standard of goodness, more or less red to some standard of redness, etc.)
Since being admits of degrees (as does goodness & every other sort of perfection), there must be something which is the standard for that attribute (a formal cause of that attribute)
That standard is God

13. Proof 5: argument from design
Natural kinds and events are only understood properly as having an end state or aim
Such non-intelligent things can act towards such ends only by being directed by something which is intelligent (something which could have motives or purposes)
Therefore there is an intelligent being which directs natural kinds and events; and that being is God.

14. Character of Aquinas’ God
What is the nature of the God which is revealed by “the five ways”?
the initiator of all motion (change)
the ultimate cause of all things
a necessary being
the standard against which all things are measured
a surpassingly complex intelligence