Are you just a "brain in a vat"
hooked up to a powerful computer?
In ordinary conversation the term 'agnosticism' usually means doubt about the existence of God, or (more strongly), the view that no one can know whether or not God exists. Philosophical agnosticism (or epistemological agnosticism) is a generalization of that view to all of knowledge; it is the theory that no one can ever really know anything, or at least that no one can know anything for certain. Why not? Supposedly because it is "conceivable" that we might turn out to be wrong about any particular piece of knowledge.
But of course lots of absurd things are conceivable. It is conceivable that you are only dreaming, and that the earth does not really exist. It is conceivable that you are a brain in a vat somewhere on a planet in another galaxy, hooked up to a powerful computer that only makes you think you are living on earth—when in fact there never was an earth. But it should be pretty obvious that such wild suppositions, while "conceivable", in no way count against our actual knowledge that the earth does in fact exist. It is not a legitimate test of knowledge to demand that you cannot conceive that something is false.
In short we do know lots of things, such as that the earth exists, that it is very old (best current estimate about 4.6 billion years), that the earth revolves around the sun, and so forth. That is to say, these things are known to science! There are of course many religious nuts and other ignorant people who deny at least some of these things.
But philosophical agnosticism is a very powerful force in contemporary philosophy and in our culture in general. Some people even imagine, quite falsely, that it is the basic principle of science. Consequently it is important to forcefully criticize this sort of craziness.
That Zeus and Athena do not exist
Unless you've visited every mountain top in Greece?
(And even then you might have missed them....)
I despise those dogmatists who say
That leprechauns do not exist.
Have they looked beneath each toadstool
In every glen in every woods?
Having not recently looked beneath my bed,
How can I be sure no bogeyman is lurking there?
I cannot prove that Lucifer is not down some deep volcano
Or that God himself is not floating out beyond the moon.
I have no evidence one way or the other
About gods and goblins, ghosts and ghoulies;
No matter how absurd the fantasy,
I must withhold my judgment.
I am so very, very scientific!
—JSH, "The Agnostic" (1988)
See also: RELATIVISM