In common understanding, agnosticism is contrasted with atheism. In the popular sense an agnostic neither believes nor disbelieves that God exists, while an atheist disbelieves that God exists. However, this common contrast of agnosticism with atheism will hold only if one assumes that atheism means positive atheism. In the popular sense, agnosticism is compatible with negative atheism. Since negative atheism by definition simply means not holding any concept of God, it is compatible with neither believing nor disbelieving in God.
Putting aside the current popular sense of the term, “agnosticism” was coined by T. H. Huxley in 1869. According to Huxley, “Agnosticism is not a creed but a method, the essence of which lies in the vigorous application of a single principle. Positively the principle may be expressed as, in matters of the intellect, follow your reason as far as it can carry you without other considerations. And negatively, in matters of the intellect, do not pretend the conclusions are certain that are not demonstrated or demonstrable. It is wrong for a man to say he is certain of the objective truth of a proposition unless he can produce evidence which logically justifies that certainty.”
It should be clear that agnosticism understood in this way does not entail atheism even of a negative sort, although it is compatible with it. Indeed, it could be compatible with theism, since some theists have argued that one can demonstrate the existence of God. Huxley’s agnosticism would entail negative atheism only if the existence or nonexistence of God was not capable of proof and it was assumed that one should not believe or disbelieve something unless it was capable of proof or disproof.
Certainly some agnostics have intended more by agnosticism than simply a methodology. On this view an agnostic is someone who neither believes nor disbelieves that a god or gods exist since their existence or nonexistence cannot be proved by reason. Agnosticism so understood is identical with one type of rationalism considered below and is compatible with a negative atheism in which belief or disbelief in God should be based on reason.
According to some dictionary definitions, an agnostic is a person who claims one cannot know whether god exists or not. This view is compatible with theism, since a theist need not base this belief on knowledge. Belief may be based on a leap of faith. So unless one assumes that one ought not to believe something unless one can have knowledge of it, agnosticism in this sense is compatible with either theism or positive atheism.
(Michael Martin, Atheism: A Philosophical Justification [Philadelphia: Temple University Press, 1990], 466-7 [endnotes omitted])