It is popular among self-confessed atheists to define atheism as ‘lack of belief in anything supernatural.’ This is handy for the atheist, since in the famous debate over where the burden of proof lies, it is a common argument to claim that the burden does not lie with the atheist, who has a mere lack of belief in God; rather, the burden is on the theist to give evidence for his / her positive assertion that there is a God. However, ‘lack of belief in God’ is a very bad definition of atheism, since it is true of rocks, camels and Christmas trees, which lack all beliefs, including about God. It would be an abuse of the word ‘atheism’ to refer to a rock as an atheist, so this definition clearly will not suffice. It is also insufficient for distinguishing atheists from agnostics, who also lack belief in God. Though it may suit the atheist in debate to characterize his / her position in terms of lack of belief, rather than a positive belief of the same type as theism, it is very hard to accurately do so. 'A belief that there is no God', which sounds more traditional, also sounds like a more accurate definition of 'atheism'.
There are so many uses of the word ‘God’ that the term is practically ambiguous. Restricting ‘theism’ to ‘Christian monotheism’ as is commonly done is too restrictive, since there exists monotheisms in many other cultures. The scale of religious conviction is a continuum, with very weak theisms positing the existence of a creating and sustaining ‘mother nature’ type force, and very strong theisms positing the existence of a single, powerful divine mind. Perhaps we might define theism as ‘belief in a creator and sustainer of the universe.’ However, this is too loose: belief in a scientific law (common to theists and non-theists alike) might well be held to fall under this definition. Should we then include creative intelligence in the definition of theism? No, since ‘intelligence’ is also ambiguous, and there are many theists who do not think of God as a giant, powerful disembodied mind (quite a crude conception), but something more abstract.
Agnosticism, taken as an ‘I don’t know’ position, is commonly criticized as being too weak, a kind of cowardly middle-man position held by the person who lacks the conviction to commit to either atheism or theism. However, the strength of an intellectual position should not be judged on the content of the belief itself, but on the amount of assessment, deliberation and criticism that has been invested in that position, and the strength to which the belief is held. Therefore, one can be a very strong agnostic if, after deliberating long and hard and weighing up the evidence, one decides that we don’t know / can’t know enough to assent to God’s existence or reject God’s existence. It's clear that agnosticism can be as firm a positive belief about the world as atheism or theism. At the very least, an agnostic must have considered the question of God's existence, if only for a second.
'I don't care'
It is also widely held that ‘atheism’, ‘agnosticism’ and ‘theism’ (or ‘religious belief’ for the purposes of this point) exhaust the possibilities concerning intellectual attitudes about God. The question ‘are you atheist, theist or agnostic?’ implies that one must fit somewhere here, perhaps on a continuum between strong atheist and strong theist. Richard Dawkins, in 'The God Delusion', introduces a similar 7-point scale. However, this is simply false, since the three categories are not exhaustive. This is because, as argued above, agnosticism is a positive belief about the world, the agnostic having actively concluded that, based on the evidence, we simply don’t (or can’t) know if God exists or not, based on at least a shred of consideration of the question. Agnosticism does not, therefore, cover the sizeable group of people who haven’t for a minute considered the question of God at all: ‘I don’t know’ is not the same as ‘I don’t care.’ He / she who has not to any extent considered God’s existence will have no belief about God’s existence, and will have come to no conclusions; therefore, to label such a person ‘agnostic’, as is popular, would be false. This person has not joined the debate; therefore such labeling is not justified. Rather than create a new term for people who have never considered God’s existence (such as ‘I-don’t-care-ists’ or some more catchy name), it is more sensible, I think, to consider such people as located outside the continuum from atheist to theist. Therefore, the three categories ‘atheist’, ‘agnostic’ and ‘theist’ are not exhaustive, and should not be used as if they are: they describe only a sub-set of human beings.