I’m not sure why this concept is so difficult for many people to grasp. Even though there are at least a hundred other blogs explaining the same topic, I guess I have to do what I have to do.
Agnosticism – anything with “gnostic” in it refers to knowledge of god(s). Knowledge in this concept is justified true belief. That is, the content of the belief must be empirically, factually true to the best of our ability to assess objective truth. Additionally, the belief must be justified. If I make an offhand guess that you, gentle reader, have exactly $1.76 in your pocket right now, and you do have exactly $1.76, I do not possess knowledge of your money. I am making an unsupported guess. If you show me the contents of your pocket, I will then have knowledge of it.
An agnostic, then is someone who lacks (by virtue of the a-, which is a negative prefix) knowledge of god(s).
Atheism – Theism is the belief in god(s). A person is a theist if they have the belief that there is at least one thing in the universe (or out of the universe) that qualifies as a god or gods. An atheist is (by virtue of the same prefix) a person who does not have a belief that there is at least one thing in the universe that qualifies as a god.
Note that one of these words has to do with belief and one has to do with knowledge. With that in mind, consider the following possibilities:
1) In factual truth, there is a god(s).
An agnostic would be someone who didn’t know that there was a god.
An atheist would be someone who didn’t believe that there was a god.
In this case, the agnostic would be ignorant, and the atheist would either be ignorant or irrational. (He would be irrational for seeing evidence of god and not believing it.)
2) In factual truth, there is no god.
An agnostic would be anyone on the planet. One cannot have knowledge of that which does not exist. Therefore, if atheists (those who do not believe there is a god) are correct in their belief — that is, if their belief also happens to be knowledge — then everyone on the planet has to be agnostic.
As you can see from this diagram, assuming there is no god, it is not possible for a theist to have knowledge of a god since no god exists. Supposing a god exists, you would just switch the two around, with “God exists” being under knowledge, and “God does not exist” being under belief.
Ok, now, on to the crux of the matter. There are two kinds of atheism typically discussed in philosophical circles — Strong and Weak.
Strong atheism – the positive statement, “No Gods Exist.”
Weak atheism – the statement of a negative, “There is no evidence for a god(s).”
These two statements are useful in a philosophical context, but in practice, it works differently. In practice, there are many things which we do not believe in, but which we cannot say definitively are nonexistent. For instance, I do not believe in gremlins, even though there is plenty of talk of them in literature and everyday gossip. ”Oh, gee, there are gremlins in my car. The damn thing won’t start every other day of the week.”
Technically speaking, I am a weak agremlinist, for I cannot definitively prove that gremlins do not exist. Practically, I am a strong agremlinist, since gremlins, as described in literature, are quite impossible based on everything we know about the physical universe. I can say that as described, gremlins cannot exist, and so I am a strong agremlinist. This is not to say that I cannot imagine the possibility that someone will come up with a definition of “gremlin” that is physically possible. If that happens, I may have to reassess my belief in gremlins. Until and unless it happens, I may remain content in my strong agremlinism.
It works just the same way with god(s). Any and all existing definitions of god (save the pantheist god, which is just a synonym for “all that exists”) are incoherent and/or impossible. Therefore, with respect to all gods that have been described, I can say that I am a strong atheist. Gods which cannot possibly exist do not exist. I hold to that as truth.
This is not to say that I cannot imagine the possibility that someone will propose a definition of god(s) that is plausible. If that were to happen, I would have to be a weak atheist until and unless someone produced even a shred of evidence that the proposed deity did, in fact, exist, and was not just a theoretical possibility.
Most atheists, then, are both strong and weak. They know that many, if not all, gods that have been thusfar invented by the minds of men are impossible, and are strong atheists with respect to them. To be philosophically correct, however, we must acknowledge the possibility that the term could become coherent in the future. Granted, we would have to be wrong about a great many things, including the foundation of knowledge itself, so there isn’t much of a chance of that happening, but since we are in the realm of induction, we must acknowledge the existence of a vanishingly small chance that we are wrong.