The world exists without having to exist; this contingency calls for a reason beyond itself, a necessary being.

Supports

Leibniz, Aquinas · Principles of Nature and Grace, §7–8 · 1714

Everything that makes up the world could fail to be: each thing owes its existence to another, living beings to their parents, bodies to their causes, and none carries within itself the reason for its existence.
Now adding up what is not self-sufficient produces nothing self-sufficient: even an infinite chain of contingent causes leaves untouched the question of why there is something rather than nothing. If everything must have a reason, the , that reason must be sought outside the series.
That reason cannot itself be contingent, on pain of raising the question all over again: it must be a necessary being, one that carries within itself the reason for its existence and could not have failed to be.

Therefore So there is a necessary being, the ground of everything contingent: what the traditions call God.

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