Recently, I've been forced to defend my atheism. That doesn't come as a big shocker to me, given that I live in the 'south' but what does surprise me is why I've been forced to defend it. It all started when a pack of youth missionaries asked me "How can you be so certain that there is no God?". I told them I wasn't. For all I know, there could be a God but I simply don't believe there is one. Like sharks smelling blood, they jumped all over that one for my apparent self-contradiction.
Such ignorance (and I don't mean that in a derogatory fashion) isn't relegated to the square walls of the coffee shop I frequent, nor does it fall within the confines of the Mason-Dixon line. Indeed, I've read several blogs and forum posts that seem to have a point confused. That point is, namely, that atheism and agnosticism are mutually exclusive. In other words, many seem to think that either you are an atheist or an agnostic - you can't be both. It would be like saying you're a married bachelor or a compassionate conservative (just kidding...sort of).
So let's get something straight: agnosticism and atheism are not mutually exclusive. One can be an agnostic atheist (hey, if I can do it, you can too). My reason for saying that is fairly straightforward - agnosticism and atheism make completely different claims regarding completely different levels of cognition.
Agnosticism (from the Greek - a (without) gnosis (knowledge)) is a claim concerning itself with knowledge, or more put more aptly, the lack of knowledge. One can be agnostic about a great number of things: the number of jelly beans in the jar, the number of cars on the road at a given time, or the true rationale for war with Iraq. In these circles, however, agnosticism usually refers to the existence of God. Someone who claims that they are agnostic when it comes to God is simply stating that they don't know. Could be. Could not be. More specifically, many agnostics (myself included) hold that it is actually impossible to know whether or not God exists.
While agnosticism makes a claim regarding knowledge, atheism makes a claim regarding belief - namely, I don't believe God exists (or you can put it into the affirmative if you prefer, e.g. I believe God doesn't exist). However you want to sexy it up, you're basically saying the same thing: we're godless creatures in a godless universe.
The key difference between these two notions is the difference between knowledge and belief. Belief is a sort of substitute for knowledge with respect to things that are not yet known or are inherently unknowable. In other words, one can believe something without having knowledge. For example, I can believe that there are one-hundred fifty four jelly beans in the jar, or that there are two point six billion cars on the road, or that the real rationale for war with Iraq was to feed the military-industrial complex.
The things one can know, however, is a bit trickier. If you really examine what knowing something means, you'll come to the conclusion that the scope of things we can actually know is somewhat limited when compared with the popular use of the word 'knowledge'. Perception can be in error. Memory can falter (see Elizabeth Loftus [1] for some interesting work on implanting false memories). We can be mistaken in our reasoning. But, of things we can know, it would seem odd to say that one believes them. For example, I know that two plus two equals four (I got an 'A' in pre-algebra). It would seem odd for me to say that I believe that is the case. Or, an old argument that St. Augustine used to trash the Skeptics - I know that I do not want to be in error. It would likewise be odd to say that I believe I do not want to be in error.
The point that I'm trying to get at here is this: belief and knowledge are different operations that are concerned with different levels of epistemic certainty. If you don't believe me...well, I guess I can't make you.
[2] | 
[3] | 
[4] | 
[5] | 
[6] | 
[7] | 
[8] | 
[9] | 
[10] | 
[11] | 
[12]