Over the years I've written numerous blog posts about the near-impossibility of proving a negative such as God doesn't exist.
This isn't the way both science and common sense work.
We don't ask for evidence that invisible fairies aren't making our cars move. Since cars have engines, or motors if they're electric, there's lots of evidence in favor of engines/motors, so no need to deal with the invisible fairy hypothesis.
Same is the case with God. Or at least how atheists look upon God.
Below is how Armin Navabi addresses this issue in his book, "Why There Is No God: Simple Responses to 20 Common Arguments for the Existence of God."
"There's no evidence that God doesn't exist."
When confronted with criticism, some theists will pull out this argument in an attempt to shift the burden of proof toward the critic. Although this tactic can feel very clever, it opens a door to absurdity.
This argument seems to suggest that we believe in everything, even things we have yet to think about, until that belief is proven false. That's simply not a logical way to perceive reality.
If the criteria for something being accepted as true was based purely on there being no evidence against it, an endless number of hypothetical objects could suddenly become "real." This has been the source of numerous playful thought experiments by skeptics around the world.
-- The flying spaghetti monster, who created the earth with his noodly appendage.
-- The invisible pink unicorn, whose "believers" logically know that she must be invisible because she has not been seen, yet have faith that she's pink.
-- The dragon in Carl Sagan's garage, a thought experiment he describes in The Demon-Haunted World. The dragon is invisible, floats in the air, generates no heat and is incorporeal, thus evading all forms of sensory detection.
-- Russell's Teapot, a hypothetical teapot that you cannot prove isn't orbiting the sun.
Of course, all of these examples were designed in good fun. Bertrand Russell does not actually believe that there is a teapot orbiting the sun. However, there is no way to definitively prove that these fanciful claims aren't true, which demonstrates the total absurdity of this line of thinking.
Ad hoc arguments
Carl Sagan's invisible dragon argument shows the futility of ad hoc arguments in explaining reality.
An ad hoc argument is one that makes excuses to rationalize away the valid criticisms of an argument without any evidence to support it. When the claimant desperately wants something to be true, she'll often employ an ad hoc argument to counter any arguments to her claim.
The dragon in Carl Sagan's hypothetical garage cannot be seen because it's invisible. A skeptic might press for evidence. But its footprints cannot be observed because it hovers in the air, and the dragon's invisible fire is heatless.
A rationalization can be formed to explain the absence of any form of evidence. These rationalizations don't make the original claim true. Indeed, it's easiest to make ad hoc arguments about things that don't really exist because that frees you up to create increasingly fanciful arguments.
When applied to theism, this ad hoc reasoning can be seen in the increasingly vague descriptions of God. The rationalizations discussed in the last chapter -- that God cannot be comprehended or described -- fall under the ad hoc fallacy.
Such rationalizations make God so vague that it becomes impossible to refute the idea, but they get the claimant nowhere closer to proving his claim.
Disbelief is not the same as belief in something else
Telling an atheist to prove there is no God automatically assumes that this is what the atheist believes. While gnostic atheists confidently believe that there are no deities, many other atheists are agnostic atheists.
In other words, these people do not believe in any gods, but they do not claim to be certain that gods do not exist. Gnostic atheists, meanwhile, do feel confident saying that no gods exist. Both are valid types of atheism.
It is important to note that atheism and agnosticism are not mutually exclusive. To understand the difference between atheist and agnosticism, visit AtheismVsAgnosticism.com
Lack of belief in deities is enough to classify someone as an atheist. Lacking belief in something does not mean that you believe it to be false; it just means that you have no conviction that it's true.
For example, a friend of yours may believe that Ford is the best car company in the world. You have no particular opinion one way of the other. You don't believe that Ford is better than any other car brand, but you also don't know that it's not the best car brand. In this situation, you would be agnostic about your friend's claim.
In actuality, most people are atheist about at least some gods.
After all, there are thousands of gods throughout the history of world theology, but the majority of religious people have no problem in disbelieving Zeus, Thor, or Anubis. Jews and Muslims have no problem denying the divinity of Christ. Monotheists are well practiced in disbelieving other gods.
As Richard Dawkins put it, atheists simply take this one god further.
Religions demand perfect evidence from anyone rebutting their claims but offer none for their own claims. If faced with convincing evidence in favor of any deity, we should reconsider our position.
But we need to ask questions and go where the evidence leads us, rather than try to lead the evidence where we like. By questioning everything, we follow the evidence, rather than trying to force the evidence to fit our presupposed conclusions.