I consider myself a rationalist, a naturalist (someone who believes in the natural world and that there is no supernatural influence) and maybe a few other such labels, but I don't let that limit my imagination. I very much like hard SF as my favorite genre, mostly because more often than other genres it expresses a rationalistic outlook on existence. But still, I've read (a moderate amount) and written (a little) in other, "less rationalistic" genres.
Mystery author Lawrence Block has a writing book, I haven't read it yet but just the title is instructive: "Telling Lies for Fun and Profit." It's what we do when we write fiction. We tell lies, the reader knows they're lies (or that there ARE lies in the story, but not necessarily what they are), but the idea is to write convincingly and interestingly enough that the reader is willing to follow along for his own entertainment.
And what Ruv said.
Here's a link to a short story I wrote a few years ago. As you'll see when you read it the MC believes, just as I do, in a naturalistic worldview, but as "The Author" I put him in a situation that challenged his belief.
If you haven't been to AW's Flash Fiction forum before, it'll ask you for a password (this is just to keep search engines from reading and indexing the writing). That password is:
flashed
Here's the link:
http://absolutewrite.com/forums/showthread.php?t=84639
I like the idea of that story, and it's kind of the thing I want to write more of, to put people in strange or uncomfortable situations like that and see how they react. As Ruv said, it's the psychology and sociology involved in these characters and situations that (help) make it interesting.