Why is there something rather than nothing?
On the axis: Existence of God · the world
The world exists without having to exist: does accounting for this contingency require a necessary being that depends on nothing, or is the world simply there, a brute fact with no explanation?
The universe exists, yet nothing seems to have required it: it might never have been. Does it then need a ground that, unlike the world, could not have failed to exist, or is the world's existence a brute fact, with no deeper reason?
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Justification
God is the supreme, necessary and personal substance, cause of the pre-established harmony and author of the choice of the best world. Leibniz proves him along several lines: the sufficient reason of the contingent universe must lie outside it; the eternal truths presuppose a divine intellect; he takes up and amends the ontological argument. This is theism in the full sense, providential and creative, not the impersonal God of , whom he opposes.
Monadology, §38-45 (God, sufficient reason, source of the eternal truths) · Theodicy, §7-8 (God, cause of the best world)
Justification
Thomas rejects both fideism, which would treat God's existence as a mere matter of faith, and 's ontological argument, which claimed to deduce it from the concept alone: for a finite mind, God's existence is not self-evident, it is demonstrated from effects. Hence the , which all start from a fact of experience (motion, causation, contingency, degrees of perfection, the ordering of ends) and rise, to avoid an infinite regress, to a first term: an unmoved mover, an uncaused cause, a necessary being. Theism is thus held to be rationally established, and not merely believed.
Whatever is moved is moved by another; we must therefore arrive at a first mover moved by nothing, and this everyone understands to be God.Summa Theologiae, Ia, q. 2, a. 3 (the first way)
Summa Theologiae, Ia, q. 2, a. 3 (the Five Ways) · Summa contra Gentiles, I, 13