What is the difference between an atheist and an agnostic? The simple answer is that the atheist denies the existence God, whereas the agnostic neither denies nor affirms the existence of God. But the line between these two is not so easily drawn. Consider the “spectrum of non-belief,” which measures the percent likelihood that one assigns to the non-existence of God. Someone with a score of 0% is a true believer in God. Someone with a score of 100% would clearly be an atheist. But what interval on this spectrum corresponds to the agnostics? Where are the dividing lines?
Interestingly, I’ve found that self-identifying atheists and agnostics both draw the lines so as to maximize the size of their own group. Thus, atheists tell me than any score of greater than 50% qualifies you as an atheist, because you think it’s more likely than not that God doesn’t exist. Meanwhile, agnostics tell me that any score less than 100% (and greater than either 0% or 50% -- I haven’t quizzed them on that end of the spectrum) qualifies you as an agnostic, because you place some probability, however small, on the possibility of God existing. Since I’m at about the 99.99%mark, I would meet both definitions, thereby qualifying as both an atheist and an agnostic.
My friend Bob proposes a sensible rule: you are an atheist if the possibility of God’s existence doesn’t actually affect your life in any concrete way. In other words, if you received definitive proof of God’s non-existence tomorrow, would your behavior change? In my case, the answer is no, because my behavior is unaffected by that 0.01% chance. But even this rule is subject to quibbling. For me, the main reason my behavior wouldn’t change is that the probability is just too small. But we can imagine other reasons for God’s possible existence not affecting you. For instance, you might think that theological ethics and secular ethics make identical demands, and so even a large change in your score on the non-belief spectrum wouldn’t make a difference. Or you might observe (correctly, I think) that believing God might exist is a far cry from knowing what he wants from you, and thus your behavior must still be guided by secular reasoning. And both of these reasons for your behavior being unaffected by the possibility of God are perfectly consistent with agnosticism or even theism.
Incidentally, here’s a great article by Daniel Dennett on defending the rights of agnostics and atheists, whom he refers to collectively as “brights.” Thanks to Radley for the pointer.