1. The ontological argument
Letter 27: God, a being greater than which none can be thought, exists in the mind and therefore must exist in reality, since a being that exists in reality is greater than one that exists only in the imagination.
2. The cosmological argument
Letters 28-29: Since nothing can be the cause of itself, there originally had to be a First Cause, which we call God. (Aquinas’s efficient-cause argument)
Letters 28-29: Since nothing can be the cause of itself, there originally had to be a First Cause, which we call God. (Aquinas’s efficient-cause argument)
3. Aquinas' first-mover argument
Letter 30: Nothing can begin to move by itself. Since things are moving, there must have been a First Mover, which we call God.
4. Aquinas' contingent-beings argument
Letter 31: Since things exist and at one time nothing existed, they had to have been created by a Being that already existed.
5. Aquinas' argument from gradation of perfection
Letter 32: Gradations exist among the characteristics of things in categories, from least to greatest. For there to be gradations, there must be an absolute and ultimate standard that is The Most Perfect Cause — that is, God.
6. Aquinas' intelligence of purpose argument
Letter 33: Things without intelligence can be seen to be working toward an end. Since they cannot know their own purpose, they must be directed by a higher Intelligence, that is, God.
7. The design argument
Letters 34-38: The world about us gives the appearance of having been designed, as a well-crafted machine is. The presence of design implies a Designer.
8. The teleological argument
Letters 39-40: If the world about us has been designed, then the presence of that design implies a purpose behind it. If there is purpose, then — since purpose cannot arise simply from material forces or blind chance — there must be an Intelligence behind it.
9. No convincing philosophical arguments against God (evil, multiple religions, etc.)
Letters 41-47: Philosophical objections to God’s existence — such as the presence of evil and the existence of many religions — are not persuasive. The failure of such arguments suggests that the opposing philosophical proposition — that God exists— is a strong one.
