The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[Belacqua]

Just to play devil's advocate, the concept of contigency is a Hegelian dialictic. Positing contingecy as a category of being implies a catagory of non-contingent being, does it not. In the same way you cannot have slaves without a master. It is not special pleading so much as begging the question by framing the necessary being demonstration within the distinction between contingent and non-contingent being.

Ooh, difficult. I like questions like this...

I see what you mean, I think. In the case of a master/slave dichotomy, each needs the other to exist. No slaves without masters, no masters without somebody to boss around. So the contingent/non-contingent dichotomy might be the same....

I'd say that it WOULD be possible for everything to be contingent, with no non-contingent thing at all, under certain conditions. Namely, if we think of the chain of causation as being temporal, and we also think that the universe is eternal with no temporal beginning point. In that case, it would be contingent turtles all the way down, and since an infinite regress is OK here, there doesn't have to be a non-contingent First Cause. You could have a world of slaves (contingencies) with no masters (First Cause). 

This is why I don't like to frame Thomist arguments as temporal chains, but rather as essential. After all, Aristotle thought that the universe is eternal with no beginning point. 

Attempts to argue a First Cause based on the Big Bang also seem to fail here, since these days they talk about new universes "budding off" of multiverses or whatever. (To what extent physicists believe this I don't know. But it's a way out of seeing the Big Bang as the beginning of everything.) 

However, if we're talking about essential chains of causality, then I think we get down to a non-contingent thing because it's necessary for existence, not simply because we need it as dialectical opposite. 

Tl;dr -- I think contingency doesn't have to exist ONLY as a pair with non-contingency, if we're talking about temporal chains.

Now here's a crazy thought: could there be a non-contingent thing with NO contingent things following on from it? It would be non-contingent, but not a First Cause, since it caused nothing. (This would be, I guess, like a God who didn't bother to create the universe.) Since the reasons why God made the world are not included in the First Cause arguments, I think this is logically possible. So there you'd have non-contingency without its dialectical opposite. 

But I've never thought about this before and I'm open to suggestion!