OK, I said in my initial post that I have more to say about why I believe what I do, but I'd get around to it later. Well, I guess it's about time I get to it. I did explain why I'm agnostic, but not yet why I'm atheistic.
I always say I'm a firm believer in science, and science almost never steers us wrong. We have some debate about this in the creationism thread, but I'd love to see somebody point out somewhere where science has led us seriously astray in the past. I'm serious: go on, do it. Bet you can't!
Now, I will say that science does not know, or really even care, one way or another about the existence of God. If finding the answer were possible, then having that answer would answer a few questions (and doubtless bring up many more), but it would not affect the principles of science itself, nor would it really validate or invalidate the majority of what we have learned.
Still, I think it is more "in line" with scientific thinking to assume that God doesn't exist until proven otherwise. There is the saying, "it is difficult to prove a negative". In other words, it is far easier to prove that something exists than something does not exist.
The problem with proving a negative is the way deductive reasoning is applied. The only way to prove something does not exist is to evaluate
all the possibilities for existence and then disprove all of them. On the other hand, proving the existence of something is much simpler: either show it to us, or build a case for its existence by piecing together the evidence. You just have to prove one thing instead of disproving many. In many cases, proving the negative is in fact impossible, for instance if there is an infinite number of possible explanations.
Because of this, the usual way of approaching things in science is to assume something doesn't exist until it's proven it does. That does not necessarily mean
asserting that it doesn't exist. For example, take the supernatural phenomenon of clairvoyance, the ability to see the future. Of course, a lot of scientists believe it doesn't exist. That's their own, personal belief, not a scientific belief, because they know that science does not preclude the possibility. But it is impossible to prove that such powers don't exist, only that a certain individual can't produce the desired result in a given test. So the only thing we can do is assume clairvoyance isn't real until somebody shows otherwise. The burden of proof is on those who say it is real.
Likewise, the burden of proof is on the religious believers and not the atheists. Of course, believers can and do point to various evidence, but rarely is it
scientific evidence. For example, the seeming improbability of a habitable planet coming together by chance, and life springing up on it also by chance, is not scientific evidence. An easy counterargument to that point is to take into account the vast size of the universe, which has a very incomprehensibly large number of stars and planets. It's hard for our minds just to comprehend the big difference between a million and a billion, and "a billion" doesn't begin scratching the surface here. There are an estimated 100 billion galaxies, and the number of stars in just one galaxy is incomprehensible to us. We're talking numbers so huge that virtually
anything that is wildly improbable, but possible, will probably happen at least once.
Likewise, anything that comes up in a religious text is not scientific evidence. If somebody successfully predicted something, good for them. How do we know it's not just coincidence? I can predict that the coin will turn up heads, and I'll be right a stunning 50% of the time!
Of course, if somebody can manage to provide me with an example of religious scientific evidence, I would be happy to debate the point.

But for now, I have no usable evidence to argue for the existence of God, so I'm inclined to believe he doesn't exist. I would not
commit to that belief, but if I had to commit to a belief, I would choose that. Well, this post is getting long like the first one, and I've said most of what I've wanted to say, so I'll wrap this up. Bring forth more debate!
- Kef