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Originally Posted by Mike F

You say that like it's an un-controvertible fact. That's one way of defining those two terms, but only one.

Most sources cited on Wikipedia for Atheism and Agnosticism agree that the common usages of the terms agree with my explanation, and, IMO, do a better job of encapsulating and distinguishing the respective ideas.

What you quoted from Wikipedia agrees with my definitions, that's why I bolded the words from your Wiki quote:

"Agnosticism is the view that the truth values of certain claims—especially claims about the existence or non-existence of any deity, but also other religious and metaphysical claims—are unknown or unknowable."

Unknown or unknowable, a position about knowledge, not belief.

Further it says: "In the strict sense, however, agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify the belief that deities either do or do not exist. Within agnosticism there are agnostic atheists (who do not believe any deity exists, but do not deny it as a possibility) and agnostic theists (who believe a deity exists but do not claim it as personal knowledge)."

Again the differentiation between knowledge and belief.

For Atheism: "Atheism is, in a broad sense, the rejection of belief in the existence of deities"

The only people who use a different definition that I know of are the people who don't know the words and simply use agnostic to indicate "I don't know."

Originally Posted by Mike F

It's unnecessarily bookish and doctrinal to insist that Atheism only means X, particularly when doing so just forces you to come up with subcategories of atheism which then need explaining in order to provide the important differentiation that the common usages provide.

I using clearer definitions for words facilitates communication, rather than being unnecessarily bookish. These are the definitions used on forums targeted towards such discussions.

The subcategories for atheism are necessary, because there are subcategories of atheists, and lumping them all together doesn't make things clearer, it makes it more obscure (as this thread demonstrates).

Subcategories of agnosticism are also necessary, because there are weak agnostics (the god question is not known because of insufficient information, but could be known) or strong agnostics (the god question is inherently unknowable no matter what, there can never be sufficient information).

The common usage doesn't provide enough because it doesn't cover all four possibilities.. one might believe but not know, one might believe and claim they know, one might not believe and not know, and one might not believe and claim they know.