So, the Atheist Bus Campaign has come to New Zealand. Once again, the promoters are being asked why the 'probably' is in the phrase.
While it would be nice - and the promoters all 'probably' believe so - to say "There is definitely no God," this runs into two issues. The first is that the 'probably' softens the blow to those not so keen on blasphemy. While I personally think that, regardless of what you believe, it is everyone's moral imperative to blaspheme - if only because we're told not to - blasphemy isn't everyone's cup of tea, so it's probably best to avoid it (even if you're only technically avoiding it).
However, given that people are going to be offended by these adverts anyway, this is a relatively minor reason.
The real reason for its existence, is that to exclude the 'probably' would be to lose scientific integrity. The question of God's existence is one that is insoluble. There is no way that you can prove that He exists, but neither can you prove a negative. This might sound a little more agnostic than atheistic, but it's not - it's an appeal to the rigours of logic. An Agnostic Bus Campaign would read more like "There's no way of knowing if there is a God or not." It's more scientifically accurate, but less to the point. Agnostics might like it, but probably won't put it on a bus. (Well, after the success of the Atheist campaign, they might...)
The atheist, like the agnostic, prefers to use the term 'probably,' but unlike the agnostic makes a point of saying that for all intents and purposes, we might as well say there is no God - for the same reason that we say that there are no faeries, no zombies, no unicorns, vampires, Thor, Quezacotl, Wotan, Zeus, Reptiods or any other mythological things. Strictly speaking, we must be agnostic to all these things (for you can never prove they don't exist), but we don't. I'm as much an aunicornist as the next person. But, ask a scientist (when they are in scientist mode, that is - not when on their coffee break) and they will say "there are probably no vampires," but that doesn't mean we need to start funding vampire hunters.
Of course, that's just my opinion. The great thing about Atheism is that every individual atheist is welcome to make their own minds up about what the word means to them and how to interpret it. I personally feel that agnosticism is a weak form of atheism and doesn't really say what you believe while tempering the wrath of the religious (in some places in the world, this is an important thing to do). But Michael Shermer thinks otherwise, as he pointed out in his book How We Believe: The Search for God in an Age of Science. He proudly uses the term 'agnostic' and he's entirely at liberty to disagree with me. That's why I love the English language - it's so vague and malleable (many people see this as a hindrance, but I don't).
So that little three syllable word continues to cause a bit of discussion, but that is the whole point in the campaign - to open discussion about the nature of the question of God's existence and how we let that question rule our lives. We mustn't hide our ignorance behind the veiled charge of 'blasphemy.'