Aquinas believes reason can establish that God exists, that God is the first cause of all things, that God is simple (without parts, composition, or potentiality), eternal (outside time), immutable (without change), infinite (unlimited by any principle distinct from God), and supremely perfect. These conclusions follow not from authority but from careful argument beginning with features of the observable world — motion, causation, contingency, degrees of being. Natural theology proves what revelation also asserts, providing an independent rational foundation for what faith accepts on God's word.
The most striking and demanding claim of natural theology, for Aquinas, is divine simplicity: there are no parts, no composition, no distinction between essence and existence in God. What God is, God is fully and without remainder. This means God is not a being who happens to be good; God just is goodness, being, wisdom, and power — not as separate attributes but as identical with a single undivided perfection. This radical claim sets the divine being apart from every kind of creaturely being and makes analogical rather than univocal predication necessary.
Aquinas is clear that natural theology reaches only a partial knowledge of God — knowledge that God is and what God is not, rather than a full positive account of the divine nature. The via negativa — knowing God by removing creaturely limitations — is more reliable than positive predications. We know God is not material, not composite, not subject to change, not finite; we say God is good and wise only by analogy, aware that these terms apply to God in a manner exceeding our comprehension. Natural theology establishes that God is; revelation begins to tell us, imperfectly, what God is.
Books I and II of the Summa Contra Gentiles develop the natural theology, followed by Books III and IV on providence and the mysteries of faith respectively.
