I sometimes hear that although an atheist may be entitled to not believe in God, he is not entitled to believe or flatly assert that there is no God. The atheist, supposedly, cannot be sure that there is no God.
It is true that lack of belief in any gods, i.e., an absence of theistic belief, is all that is required to make one an atheist. I agree with George H. Smith there. But any atheist, if he comes at the question with the appropriate epistemological greaves and cuirasses, which consist in nothing more than uncomplicated and explicit acceptance of the self-evident and inescapable axioms at the base of all thought, is also fully justified in asserting that there is no God. Not only has the burden-of-proof requirement never been met by proponents of notions of supernatural entities of any sort, but it is impossible for this burden of proof to ever be met. How can there be “evidence” for the existence of entities that cannot possibly exist? One is entitled to believe and to positively assert that impossibilities do not exist and cannot exist.
In addition to the theist’s claims about God’s impossible powers (e.g., although theists often quarrel about points fine and not so fine, God is said to be able to violate the law of identity via miracles; is said to be omniscient, omnipotent, creator of the universe, etc.), God and any gods are the kinds of entities for whose existence no evidence is ever provided. (Storytelling is not evidence, so no need to “contradict” me by pointing to Homer or the Bible.)
If somebody were to arbitrarily assert that there exist winged elephants (elephants of the bulk and weight we we are acquainted with, using wings to fly with), and I say that I don’t believe in winged elephants, then I would be an a-winged-elephantist, a non-believer in winged elephants. I am such now. I am also sure that winged elephants don’t exist. (I am sure only because I have paused to consider the notion; normally it would not come up and I would not be expending any epistemological effort to assess it.) The proposition contradicts everything we know and cannot be substantiated by any direct or indirect evidence.
Now, if (actual) evidence could be provided for a winged elephant, rendering the claim of the existence of winged elephants non-arbitrary, I would have to examine that evidence to come to a judgment about what if any value it has. This evidence would have to show that under certain natural conditions, a certain kind of creature could and evidently does exist (perhaps in another star system) that I would be willing to call a “winged elephant” because of its similarity to elephants and the similarity of the unexpected limbs in structure and capacity to wings. But this new evidence, requiring a new conclusion in light of my new knowledge, could not be of the sort that stipulates that winged elephants are invisible, or “beyond human understanding,” or capable of contradicting the nature of things, etc. I can be shown actual evidence for winged elephants only if the winged elephants are part of the natural world. Thus they would have a specific, finite nature, interacting with other things in nature and having some kind of effect on those other things.
To be is to be something, a something that cannot be what it is and not what it is at the same time and in the same respect. If the entity that men call God, an entity currently invisible and imperceptible, were in fact a part of nature; if this “God” did in fact have a specific identity that could not be violated and this “God” could not wield magical identity-violating powers, etc., then it would not be “God” or a god in any familiar sense of the term. The entity would be a part of nature, interacting with other parts of nature, leaving evidence of its existence, footprints and so forth. But it would not be able to alter any part of the universe (let alone bring it into existence) by saying things like “Let Their Be Light.” For the entity to secure effects, it would have to enact causes. There would have to be a light bulb or a candle involved, or at least flint or lightning.
The God we’re told about is not only implausible, but impossible. What we’re entitled to take as self-evident starting points of thought are the facts that the things that we perceive exist, that they are what they are, that they act in accordance with their nature. Any claim about the existence of an entity whose very nature is supposed to be exempt from the constraints of nature is an impossibility. I know that impossible things can’t exist. Not only is there no God, but I also know that there isn’t.