I don't think I've seen this before. It's an old piece in Time Out, around the time Synecdoche, New York was released, in which Charlie talks about Synecdoche, "fun" movies vs. Charlie movies, and not knowing whether he exists. The entire article is one long Charlie Kaufman quote--9 paragraphs, pieced together by interviewer Tom Huddleston--and it's a good read.

Part of life is the struggle to try and find meaning. It’s not unique to a writer. I guess there’s a certain futility in it, in that we know so little about what’s happening in the world. I mean that in a profound way: I don’t even know that I exist, let alone what’s happening.There are so many questions and there’s so much confusion and limitation in the human brain, in the same way as in a dog’s brain. We can see clearly that there’s so much that a dog will never understand, ever, no matter how much you teach him. Just move up a couple of steps on the chain and you have to come to the conclusion that we’re pretty much in the dark in a cosmic sense. We don’t know what’s going on, what reality is, and even if there is such a thing. (Source)

Additional information