From an article on him in the latest New Yorker:
It (his latest book, the third in his Dust series) has something to do with consciousness, but I didn't explore that fully, and I'm using this story, among other things, as a way of finding out what I mean by this idea.
And you find out as you're writing?
Yes. For me, it's got to be that way. I couldn't possibly write a novel if I had to work it all out first. I'm writing into darkness, as it were, not knowing where the story is going or what the characters are going to discover. It's more exciting like that. I would just be too bored—terminally bored—if I knew everything in advance.
That's how I write too. I generally a specific destination in mind, so I write for the ending, but I leave the specifics for later—though that approach often means writing the entire first draft sequentially, as you don't know the details until you write them down.