St. Thomas Aquinas' Arguments for God's Existence

AquinasÆs five basic a posteriori arguments for the existence of God (as summarized in Summa Theologica, Part I, Question 2, Article 3) are as follows.

First is the argument from motion, which he attributes to Aristotle, according to which all motion must ultimately be caused by an unmoved mover.

Second is the argument from the nature of an efficient cause, that is, by progression from contingent beings, which cannot cause their own being, to the being which needs no cause because its essence is existence itself.

Third is the argument from possibility and necessity, that is, from beings whose existence is only possible but not necessary to the one being whose existence is necessary and for whom nonexistence is not possible.

The fourth is the argument from imperfection, that is, from the imperfection of material objects to the absolute perfection of the divine. This derives ultimately from PlatoÆs argument for the existence of perfect ideas in contrast to the imperfections of material objects.

The fifth is the argument from the ôgovernance of the worldö or from order, that is, from th

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the first of these. The first way is as follows (Aristotle, Physics, VII, 1; 241b 24). Everything that moves is moved by something else that moves it. This mover is itself either moved or unmoved. If it is not moved, then this unmoved mover is the desired conclusion: that one must posit an unmoved mover. This, says Aquinas, we call God. If it is moved, then it must be moved by another mover. Consequently, the argument must either proceed to an infinite regression of movers or else it must conclude with an unmoved mover. Aquinas, like Aristotle, asserts that it is not possible to proceed to infinity and concludes that there must therefore be an unmoved mover. In the preceding argument, Aquinas explains, there are two propositions that must themselves be proved. These are (1) that everything that is moved is moved by another; and (2) that in movers and things moved one cannot proceed to infinity. Aquinas says that Aristotle proceeds to prove proposition (1) in three ways. (a) First, Aristotle argues that, for the purpose here, one must be concerned with something that is a primary mover, that is, something whose whole self moves its whole self, not one in which one part moves another. Aristotle also says that a self-moving
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