Illustrating the burden of proof - pay me!

[Belacqua]

I've always been curious about that... so it's not the case that you see Aquinas' arguments for instance, for the necessity of God, as sufficient reasons in the sense of the PSR, for the actual existence of God (in the why sense, if not the how sense)? That's how I assumed you might be squaring that circle, but looks like I was wrong on that, ie that there is still a question there for you as to how God exists, if not why?

 
I wonder if here you've unintentionally done a slight side-step between two meanings of the word "reason."

It sounds as if you're using it (as I did earlier) to mean "the justification we have in our minds for believing that something is true."

But I'm pretty sure that the Principle of Sufficient Reason doesn't require such a mental concept. It holds that for any contingent event or object, there must be some reason for it to happen or exist. However, this reason may be unknown to people, or even be unknowable by us. It might be clearer to think of "reason" here as the full set of the things' causes and history. 

There's a reason why I found somebody's gold tooth in my garden, but I have no idea what it is. I don't doubt that the tooth got there in some non-magical way, with a full set of real-world events leading up to it being there. But I'm stumped as to what they are. (I dug a little deeper thinking I might find all the rest of my landlady's late husband, but was not rewarded.)

So we wouldn't say that Thomas's concepts constitute sufficient reasons in the sense that the Principle uses that term, although an individual might or might not think they are reasons (mental concepts justifying belief) to believe in God. In fact Thomas would argue that there is no reason for God's existence, because God is not contingent. 

In other words, I don't think a Christian is invoking the Principle of Sufficient Reason when he claims he has enough reason to believe in God. He might, however, claim that because everything that is contingent requires a sufficient reason for its existence, that this is a justification for believing in God.