The to timeless question “Why is There Something Rather Than Nothing?”, one of the hallmarks of classical theism in the various expressions of the cosmological argument is the concept of metaphysical necessity, to Aristotle’s logic that there must be a First Cause (an UnMoved Mover or Uncaused Cause), or else nothing would exist.
Expressed in a variety of different ways by Aristotle, Aquinas, and Leibniz, with the problem of change, principle of causality, and principle of sufficient reason applied to existence itself, the focus is on change/Unchanged, caused/Uncaused, contingent/Necessary. In these arguments, God is Being Itself, the UnMoved Mover, First Cause, and metaphysical Necessity of which all contingent things depend, or else nothing would exist. Aquinas, in his Five Ways, the the first three ways (argument from motion, argument from causality, and argument from contingency) all effectively arrive at this principle. There’s an important point here though. God is applied at the end of the argument; in Aquinas’ words, “and we call this God.” One could of course have take issue with the word “God”, but they’d have confront the the problem of change, principle of causality, and principle of sufficient reason – on how a constantly changing, contingent existence is its own source.
In my recent discussion with Adam Lee and Massimo Pigliucci, Massimo challenged both Adam and myself on our respective positions (as a good philosopher should). Naturalists, with the arguments made above, I’ve typically seen reply in three ways;
 Denial that the question has any meaning (which is Adam’s position).
 The logic above is followed save the “and we call this God”. The UnCaused Cause, UnMoved Mover, or Necessity is a natural one (perhaps a Platonist solution, which makes for a curious naturalism) — Adam’s backup position when pushed by Massimo.
 The universe itself is a “brute fact” (a Humean causal skepticism is employed, where there is no need for a First Cause).
In many cases, some or all of these are put forward. Bertrand Russell, for example, in his famous debate with Frederick Copleston, offered both  + . Massimo put forward Hume’s skepticism on causality as a response to Aristotle and Aquinas – that we can have a sequence of events (that seem to have a necessary cause), but that “causes” are just human creations. If this is the case, perhaps the universe is a “brute fact”, not only a fair counter to Aristotle and Aquinas. Or, perhaps metaphysical necessity is granted, we just don’t call it “God”?
Certainly, one could claim the principle of causality is false (good luck with that one) or is true for everything except the universe/multiverse (a “brute fact”, biting the bullet on an infinite regress) or is true (either “God” or some natural source that qualifies as an UnCaused Cause or UnMoved Mover). Either way, the concept of metaphysical Necessity, I argue, must at least be dealt with thoroughly by theists and naturalists.