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The first three of Aquinas' Five Ways seem to be very similar to each other. They can be generalized into one argument and they all rely on the impossibility on infinite regress and argued that God is the first cause/unmoved mover where the infinite regress stop. Why did he dedicate three of his five ways to essentially the same argument?

Was he hoping to convince people of at least one of them? I can't imagine why one would accept one or two but not the other(s). On the other hand, the criticisms of Aquinas' Five Ways (possibility of infinite regress, identity of the First Cause) apply equally to all these three, so having three different arguments doesn't help there.

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  • I would tend to agree strongly with one point in Jo's answer -- which is that while all three arguments use reductio to bad infinite regress, Aquinas does not intend them to be identical.
    – virmaior
    Apr 13, 2016 at 8:15

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Part 1, question 2, article 3 from Aquinas, Thomas: Summa Theologiae enumerates five ways to prove the existence of God:

1) God as the first mover (primum movens)

2) God as first cause (causa efficiens prima)

3) God as necessary by himself (necessarium per se)

4) God as cause of being, and goodness (causa esse, bonitatis, perfectionis)

5) God as the designer of the ends (causa finalis).

The first way is taken from Aristotle’s Physics, book VII and VIII. The second way is taken from Aristotle’s Metaphysics, book II. The third way is similar to a reasoning from Aristotle’s Metaphysics, book XII. But Thomas changes the argument supporting the claim that not everything passes away.

I agree, inductive reasoning and starting the chain of reasoning with a first cause is common to all five ways, not only to the first three.

My conclusion:

  • Thomas did not aim to generalize all "proofs" to the abstract principle of the impossibility of an infinite regress. Instead, his aim was always to recall explicitly as many arguments as possible from Aristotle. Because Thomas wants to show that a huge part of Aristotle’s philosophy (reason) justifies the argumentation from theological statements (faith).

  • In addition, if people are not used to the abstract level of principles, possibly one can convince them by appealing to specific and concrete examples of a principle in question.

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  • +1. The arguments are distinguished by the different central concepts invoked: motion, efficient causation, necessity, goodness and acting for the sake of an end. I would quibble with the claim that Aquinas wants reason to "justify" faith. That's putting it too strongly---Aquinas wants to show that faith is reasonable; nothing that the faith says is provably false. But he doesn't think that everything that you should believe, such as the doctrine of the trinity, or the incarnation, can be proven by philosophy. See the very first article of the ST (1a, q. 1, a. 1).
    – user5172
    Apr 13, 2016 at 11:25
  • @shane I completely agree with you. I wanted to say: reason justifies the argumentation used by faith, e.g., manifestum est probationes quae contra fidem inducuntur, non esse demonstrationes, sed solubilia argumenta (ST I,1,8)
    – Jo Wehler
    Apr 13, 2016 at 11:35
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Another use of proofs of God's existence is to explore aspects of who God is. For example two examples of why this would matter (to Thomas or other theologians):

  1. In the "Unmoved Mover" proof, by talking about potentiality and actuality, we see God as the thing that brings actuality to everything, and as Thomas says ("nothing can be reduced from potentiality to actuality, except by something in a state of actuality"), God is in a state of actuality for all things. When Thomas gets to the transcendentals (Truth, Goodness, Beauty), he can then identify God with the transcendentals because God is the ultimate actuality of them (i.e., God is Truth, etc.).

  2. In the "From Contingency" proof, we see that God does not require anything (his existence is not contingent on anything). In particular, this means that he didn't need to create the universe, and that creation was therefore the ultimate act of love (because it was something done for its own sake, not for God's own).

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