Summa Theologiae 1.2.2

Article 2: “Whether it can be demonstrated that God exists?”

If we are to know that God exists (and not just believe it as a matter of faith), a demonstration must be provided. This of course doesn’t necessarily mean, however, that there is a demonstration for God’s existence. It could turn out to be the case that God’s existence is simply a matter of faith. Reasoning from the Apostle’s Paul’s claim in Romans 1 that God is clearly seen in the things that he has made, Thomas concludes that we can know by the power of human reason that God exists. Notice Thomas has cited biblical authority in defense of reason’s authority. The Bible itself testifies to reason’s basic reliability and legitimacy.

In the body of the article Thomas distinguishes two ways in which you can prove something:

i.      a priori demonstration: proving an effect through its cause.

a posteriori demonstration: proving a cause through its effect. It’s in this sense that we prove God’s existence, by looking at his effects and inferring from them what kind of cause would be necessary to produce these effects.

 

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