Summa Theologiae 1.2.3

Article 3: “Whether God exists?”

Having shown that God’s existence isn’t self-evident (article one), and that it in principle can be demonstrated (article two), Thomas at last shows us some of the different ways it has been demonstrated. Here Thomas gives us his famous five ways for proving God’s existence (though none of these were unique to Thomas in his day). Note that these are five ways of proving God’s existence, not the proofs themselves. He is effectively saying, “If you are going to prove God’s existence, here is how you might go about doing it.”

A summary of the five ways (Peter Kreeft gives a nice analysis of these in his notes in his Summa of the Summa):

  1. First way is from motion: very Aristotelian.
    1. Thomas calls this the “most manifest” way; notice that he begins with the senses.
    2. Summary: Things are in motion; things are set in motion only by a mover which itself must have been set in motion; this can’t go on to infinity, therefore there must be some first mover.
    3. Conclusion: God is the Prime Mover.
  2. Second way is from efficient causality.
    1. Again, Thomas appeals to the “world of sense.”
    2. Summary: We find an order of efficient causes which can’t go on to infinity, therefore there must be a first cause.
    3. Conclusion: God is the Uncaused First Cause.
  3. Third way is from the question of possibility and necessity.
    1. Again, “We find in nature things…”
    2. Summary: What we find in nature are possible things, which is to say, contingent things, things which don’t have to but merely happen to be; if this were the case for everything, however, then there would have been a moment in eternity past when nothing was. But nothing can follow from nothing, and so there must always have been some being, which is to say, a necessary being and this is God.
    3. Conclusion: God is the one Necessary Being.
  4. Fourth way is from gradations of being: very Platonic
    1. gradation is “found in things”
    2. Summary: Things admit of a more and a less, but this requires that there be something which is best and most in being. This is God.
    3. conclusion: God is perfect, complete Being.
  5. Fifth way is from ordered governance (more Stoic—see Balbus’s discourse in Cicero’s On the Nature of the Gods)
    1. Summary: all things, including those which lack intelligence, nevertheless normally act so as to obtain the best result. There must therefore be something guiding things in this way, and this is God.
    2. conclusion: God is an intelligent, ordering Mind.
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