1225 – 1274
(Aquinas notes created by Kevin Vallier)
• Dominican monk, born to Italian nobility.
• Worked ~150 years after Anselm.
• Student of Albert the Great
•Studied and commented upon much of Aristotle’s works soon after
their translation into Latin in western Europe
•Studied important Islamic philosophers including Avicenna and
Averroes, who themselves were heavily influenced by Aristotle
“Granted that everyone understands that by this word “God”
is signified something than which nothing greater can be
thought, nevertheless, it does not therefore follow that he
understands that what the word signifies exists actually, but
only that it exists mentally. Nor can it be argued that it
actually exists, unless it be admitted that there actually exists
something than which nothing greater can be thought….”
(Summa Theologica I.II.I)
• Anselm’s argument rests on his definition of god as
“that which nothing greater can be conceived.”
• Aquinas rejects the a priori Ontological argument on
the grounds that a word’s meaning cannot alone establish
the existence of that to which the word refers.
• Rather, Aquinas requires existence proofs be empirical/
a posteriori, ie. to require observational evidence.
(1) Cosmology = the empirical study of the universe considered
as a whole system
(1) Eg, Hubble’s (1929) discovery that the universe is
expanding rather than in a steady or static state
(2) Eg, Einstein’s (1915) identification of gravity the curvature
of space/time
(3) Eg, The “big bang” theory of the origin of the universe
(2) A cosmological proof for the existence of God derives
God’s existence from facts established by cosmology
• Form of a cosmological argument:
• By observation of the universe or parts of the
universe we know a posteriori that the universe or
parts thereof have property P
• The best explanation of P is the hypothesis that God
• Therefore, God does exist
Argument from motion
Argument from efficient causation
Argument from possibility and necessity
Argument from gradation
Argument from governance
1. There is motion.
2. Motion involves the reduction from potentiality to
actuality. (eg: an activated spring on the garden gate)
3. So all motion was first potentiality.
4. Only what is actualized in some regard can reduce
something from potentiality to actuality in that regard.
(eg, the squirrel that releases the coiled spring on the
5. Nothing can reduce itself from potentiality to actuality.
6. Nothing can move itself.
7. There cannot be an infinite sequence of movers.
 There must be an unmoved prime mover, i.e. God.
1. The sensible world is full of the effects of
efficient causation.
2. Nothing can be an efficient cause of itself.
3. If a thing’s cause is absent, then it cannot exist.
4. Efficient causes are ordered: 1st ->
intermediate -> (ultimate) effect.
5. So without a first cause, there cannot be
(ultimate) effects.
6.  So there must be a first efficient cause, i.e.
1. The arguments from motion and efficient causation
deny that anything can move or cause itself – except
God. However, what establishes this exception?
Rather, why should we not simply say that the
sequence of causes/movers is historically infinite?
2. Also, Aquinas’ arguments do not demonstrate that
• a single thing is responsible
• the first cause in any way resembles the JudeoChristian god
• the thing responsible still exists
1. Some natural things exist contingently.
2. For each contingent thing, there is a time at which
it does not exist.
3. If everything were contingent there would be a
time in the past when nothing existed.
4. If there were such a time, then nothing would
now exist.
5.  So, something must exist that is not
contingent but rather necessary and which gives
rise to all things. This necessary being is God.
• Even if each particular object in the history
of the universe exists contingently, it does
not follow that there must be a time when no
contingent objects exist.
1. Everything is comparable /commensurable with
respect to value and being.
2. There is a gradation in these properties, especially
value. That is, some things are better than others.
3. One thing is better than another only if the former
is more similar to that which is perfect than is the
4. So, since things are comparable with respect to
value, there must exist that which is perfect, and
this can only be God.
1. The third premise is critical to the argument.
However, this premise is dubious. What reason
secures that variation in value presupposes the
existence of the standard of perfection? Notice
that there are many examples of comparability
that do not presuppose the existence of the
relevant standard: e.g. one thing can be taller
than another without it being the case that there
exists any (third) object on which the
comparison depends.
1. Some natural unintelligent objects have
purposes. (E.g. the purpose of the heart is to
pump blood; the purpose of the wing is to enable
2. Purpose is always the result of intelligence.
3. The purpose of some natural unintelligent objects
was not established by any natural intelligent
being (such as a human).
4.  So, there must be a supernatural intelligent
being responsible for such purpose, and this is
Is it correct that objects have purposes in the
sense required by the argument?
If objects do have purposes in the sense
required by the argument, might these
purposes not be established simply by the
unintelligent processes of evolutionary

Aquinas - University of Arizona