When I got to college, I met someone who called himself an agnostic.  He said he was so filled with doubt that he did not have any belief about the existence of God one way or the other. It was then that I realized there was a distinction between not believing God exists on the one hand, and believing that God does not exist on the other.  From this I concluded that:

A theist believes God exists.

An atheist believes God does not exist.

An agnostic has no belief about God’s existence either way.

Given these definitions, a stone would be an agnostic, for having no beliefs at all, a stone has no beliefs about the existence of God.  Therefore, we must restrict our consideration not only to people, who are capable of having beliefs, but also to people who have some idea of God and have thought about whether he exists or not.

So understood, these three concepts were mutually exclusive and jointly exhaustive of the ways in which one could have or not have a belief regarding God’s existence.  The distinctions were clear and easy to understand.  All was good.

Somewhat later, I met another person who said he was an agnostic because he did not know whether there was a God.  I was not sure what to make of this at first.  Up till then, I had understood these concepts in terms of belief, and here this person was introducing knowledge into the subject.  Now, sometimes people say they do not know something when all they mean is that they have no opinion on the matter.  For example, assume someone asks me, “Did Bob ever get married?” If I say, “I don’t believe he did,” I will be understood as saying that I think Bob is still single.  The syntactical form of my answer may make it appear that I deny having a belief, but the meaning conveyed will be that I do have a belief, and it is the belief that Bob did not get married.  To indicate that I have no belief one way or the other as to Bob’s marital status, I must answer, “I don’t know.”  In like manner, it might have been that this person was using an expression about not knowing only to indicate a lack of belief.

On the other hand, it could be that he meant to say something more than just what he did or did not believe, that he was stating an absence of knowledge as his reason for counting himself an agnostic.  In that case, he was saying that an agnostic is someone who does not know whether there is a God.  But this throws things out of kilter.  Given that sense of the word, the three terms no longer neatly partition belief or lack of belief about God; for it might be that no one knows whether there is a God, in which case everyone is an agnostic, from the most devout fundamentalist to the most militant atheist, from the Pope to Richard Dawkins.  Any definition of agnostic that includes everyone is too broad.

Perhaps we should amend this to saying an agnostic is someone who does not claim to know whether there is a God.  But that is still too broad a definition. I do not doubt that there are atheists who claim to know there is no God, and I have known a few religious people who claim to know there is a God, but most people make neither claim, whether they are theists or atheists.  In other words, this definition of agnostic gives the result that a lot of theists and atheists are also agnostics, provided they make no claims about knowledge regarding God.  But this is not the way most people understand these terms.  Whatever else these words may mean, we expect their meanings to be mutually exclusive.  But when “theist” and “atheist” are defined in terms of belief, while “agnostic” is defined in terms of knowledge or claims about knowledge, there will be substantial overlap, and the presumed mutual exclusivity will not hold.

Similar consideration applies for those who, like T.H. Huxley, emphasize claims about certainty rather than knowledge.  We get the same problem, which is that theists and atheists can be considered agnostics provided they make no claims about certainty, even though most of those same theists and atheists would take exception to being so classified.  In any event, we need not linger over Huxley.  He may have popularized the word, but I am more concerned with how the word is used today.

Until now, I have considered only those definitions of “agnostic” that apply to an individual.  That is, my definitions have been of the form, “an agnostic is someone who…,” followed by “does not believe…,” “does not know…,” “does not claim to know…,” or “does not claim to be certain….” But some definitions go beyond the individual and extend to all of mankind.  In such cases, it is “agnosticism” rather than “agnostic” that gets defined.  In particular, one definition of agnosticism is the doctrine that God’s existence or nonexistence is unknown.  In other words, it is the doctrine that nobody knows whether God exists.  And another definition is even stronger, claiming that God’s existence or nonexistence is unknowable, thereby making a claim not only about how things stand today, but for all time.

The traditional definition of knowledge is justified true belief. Let us assume just for the moment that God exists.  That means the second condition for knowledge of God’s existence has been met:  the proposition that God exists is true.  A theist is someone who believes that God exists, and thus the third condition has been met for him.  Therefore, it must be the first condition where the problem lies as far as agnosticism is concerned.  That is, according to this doctrine, the theist is not justified in believing that God exists.  The same holds for the atheist if we assume that God does not exist.  In that case, what the atheist believes is true, so it must be that he is not justified in believing there is no God, if agnosticism understood as denying knowledge in this area is correct.

In other words, if the agnostic were to allow that a theist is justified in believing there is a God, and if it is true that God exists, then all the conditions for knowledge will have been satisfied, and it will follow that the theist knows that God exists. Likewise, if the agnostic were to allow that an atheist is justified in believing that God does not exist, and if it is true that God does not exist, then all the conditions for knowledge will have been satisfied in this case, and it will follow that the atheist knows that God does not exist.  Therefore, in order to assert that no one knows whether God exists, the agnostic must maintain that no one is justified in believing in God’s existence or nonexistence.  Now, to say someone is not justified in having a belief is to say that he ought not to have it, because it is foolish to go around believing stuff without any justification.

This results in a paradox.  At first blush, it appears that agnosticism is a modest, humble position, simply making no claims about God’s existence. But now we find that on this interpretation of agnosticism, it appears to be rather contentious, for it asserts there is no justification for believing that God exists or for believing that God does not exist.  And if such beliefs are not justified, then there is something inherently wrong with being either a theist or an atheist, quite apart from the question of whether there is a God. And this is a far cry from the kind of agnosticism we considered in the beginning, defined solely in terms of belief, in which an agnostic might say, “I have no opinion about the existence of God, and everyone is entitled to believe whatever he wants.”

Thus we see that agnosticism can range in meaning from mere lack of belief regarding the existence of God to an assertion that God’s existence or nonexistence is intrinsically unknowable. The former allows the terms “theist,” “atheist,” and “agnostic” to partition the possibilities of belief regarding God; the latter does not.  The former is a mere statement of one’s lack of belief; the latter impugns the beliefs of others as not being justified. Either of these meanings can be embraced, and the corresponding positions consistently maintained. Unfortunately, these different meanings are not always carefully distinguished.