Steven, it's simply a logical convenience to divide a the universe into two groups. I can always say that there are two kinds of people: those with characteristic A, and those who do not possess characteristic A. You cannot straddle the fence. Since a belief is a positive thing, the absence of belief is simply defined as the logical negation of belief. In that sense, a rock, an infant and a dead person are atheists (lacking in belief). Now the way you phrased the issue is subtly different, because disbelief is a positive attribute (as opposed to the absence of belief which is a negative attribute).
Now to really muddy the waters, you can have an agnostic theist. That is a person who believes in a supreme being but hasn't the vaguest idea of what the characteristics of that being may be. This is a truly confused individual, because virtually all believers will define a supreme being in terms of omnipotent, omniscient, benevolent, etc. But these universally accepted characteristics of God raise some huge paradoxes that all religions seem to grapple with, and in the end punt. Maybe we can talk about those later.
TTFN, CTC |