a contingentia mundi

What need not exist cannot and would not unless made to do so by something that cannot not exist.

Alternatively, only what cannot not exist can exist without being made to do so by something other than itself that cannot not exist.

A crucial premise in any argument for the existence of God a contingentia mundi.

Contemporary atheism supposes it is false, and that to the contrary contingent things, a whole world of contingent things, or a contingent (possible but not necessary) world can exist without being made to do so by anything necessary.

(Some put the point by saying existence is de trop or absurd.)

Or at any rate they refuse to believe otherwise absent proof.

But if you can prove everything you're going in a circle, someone wise once said.

And anyway nobody can prove most of the philosophically interesting - indeed crucial - claims by which we live.

People who go in for that sort of thing might even claim the thing is epistemologically basic.

Another age would have said self-evident.

Plantinga once suggested the existence of God, itself, be regarded as epistemologically basic.

That might be a bridge too far.