What is the Leibniz contingency argument?

Why is there anything at all? Leibniz posed the question in 1714 and thought its answer was God. His argument from contingency is not the kalam — it does not depend on the universe having a beginning; even an eternal cosmos would still need explaining. It asks not when the universe began but why it exists rather than not.

In Craig's formulation: everything that exists has an explanation, either in the necessity of its own nature or in an external cause; if the universe has an explanation, that explanation is God; the universe exists. A contingent being could have failed to exist; a necessary being exists by its own nature. The universe looks thoroughly contingent — it points beyond itself to a necessary, transcendent ground.

The sharpest reply is the 'taxicab' charge: if everything needs an explanation, what explains God? But premise one never said everything has an external cause — only an explanation, either external or in its own necessary nature. God is not an exception to the principle; God satisfies it, as the being whose explanation lies in his own nature. Graham Oppy presses that the cosmos itself might be the stopping point; but a universe that could have had different laws, or not existed at all, wears its contingency on its sleeve, while a necessary ground does not.

Suppose it succeeds. It shows there is a necessary, self-explanatory ground of all contingent reality — a great deal, though not yet the personal Father of Christian worship; that takes further steps. It is one strand in a cumulative case. Leibniz's child still asks why there is anything at all — and a necessary being remains one of the most serious answers a thinking person can weigh.