Thomas Aquinas provides five proofs for the existence of God through reason alone. The five proofs are: 1) The argument from motion, which argues that all change requires an initial mover which is God. 2) The argument from efficient cause, which argues that all effects must have a cause and there must be a first cause which is God. 3) The argument from possibility, which argues that all contingent beings must have a necessary being as their source, which is God. 4) The argument from gradation, which argues that all qualities must have a maximum standard, which is God. 5) The argument from design, which argues that purposeful order requires an intelligent designer, which is God.

1. Aquinas’ Proofs
The five ways

2. Thomas Aquinas (1225-1274)
 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