The most common impression people seem to have is that agnosticism is a middle-ground position between theism and atheism, and even though in practice this pretty much does turn out to be true, strictly speaking it’s not correct. That is, agnostics do almost always fall in between theists and atheists (at least on the traditional, narrow meaning of “atheist”) but agnosticism is not defined as this middle-ground position. As a result, there are other possibilities.
Usually, agnostics claim that no one knows whether there are any gods – or even that knowledge in this area is impossible for human beings. Agnostics also typically argue that, because no one really knows, everyone should suspend judgment on the question of God’s existence. Thus, they usually neither believe nor disbelieve, and in this sense, they do occupy the middle position between theism and traditional atheism.
However, it is certainly possible for someone who believes in God to hold that no one really knows whether there is such a being. Conversely, it is also possible for someone who believes that there is no God – someone who positively disbelieves – to say that no one really knows. In both cases, even though these individuals may not apply the term to themselves, they in fact are agnostics: the first is an agnostic theist, the second an agnostic atheist.
Theism and atheism are about what we believe rather than about what we claim to know. A theist believes there is at least one god. The definition of atheist is a bit trickier. Traditionally, an atheist was thought of as someone who positively disbelieves, but in recent times it has become more common for people to consider anyone who lacks belief to be an atheist. (See “Broad vs. Narrow Atheism” below for more on this.) Rather than there being only some agnostic atheists (as described above), on the broad definition just about anyone who calls himself an agnostic is also an atheist. To put it another way, on this broad view atheism is equivalent to nontheism, so that anyone who is not a theist (and hardly any agnostics are) is an atheist.
Another source of confusion, as already mentioned, has to do with the meanings of such words as “knowledge” and “belief.” Now, even though the meaning of the former, in particular, is somewhat controversial (at least among philosophers), we all use it in perfectly non-controversial ways all the time. For instance, we all would claim to know who the current president of the United States is. In discussions over atheism and agnosticism, however, people tend to use the term in a much stricter sense. Often, they claim that if you aren’t one-hundred percent certain there is no God, or even that if you can’t disprove the existence of God, then you cannot claim to know God does not exist. But why claim that knowledge requires certainty? You know that Obama is president, but can you be 100% certain? Maybe he was assassinated five minutes ago and you haven’t heard about it yet. (And if, by some amazing coincidence, such a thing does happen right before I post this, the Secret Service will probably be knocking on my door sometime over the next few days.) The point is that knowledge does not mean absolute certainty. One can claim to know that God does not exist, just as one can claim to know that unicorns and the Loch Ness monster do not exist, without implying that one cannot possibly be wrong.
Finally, belief is not – as some atheists appear to suppose – equivalent to faith. There are both justified and unjustified beliefs, and there is nothing irrational about holding the former type. In fact, everything that one knows is also something that one believes.