The transcendental argument for the existence of God (TAG). Wikipedia defines the argument as follows:
- "The Transcendental Argument is an argument for the existence of God that attempts to show that logic, science, ethics (and generally every fact of human experience and knowledge) are not meaningful apart from a preconditioning belief in the existence of God."
That is, knowledge cannot be obtained absolutely unless the source of that knowledge is itself an absolute source (read: being/God). Therefore, either you subconsciously believe in an absolute being that upholds and makes absolute the laws of the universe/morality or you do not—and can not—know anything for certain.
So what? Many non-theists, when they are backed against the wall, will admit that they know nothing with 100% certainty. Humans generally will prefer some explanation rather than no explanation. However, providing "some explanation" does not make the claims in the explanation true. Absolute certainty is in general meaningless as by definition one would have to be omniscient to acquire it. Atheists do not in general make claims to the absolute truth of things; this is usually the domain of the theist.
Examples of things that some may call absolute certain is the idea that the sun will rise tomorrow. To be truly absolute certain, you would have to know the future to know that it indeed will rise. However that is in general useless. Its much more accurate to say that we have evidence that for quite some time now the sun has risen in the morning, so we can be reasonably certain that it will do so again tomorrow.
Some claim that TAG employs circular reasoning. However, there are counter-counter-arguments to this.
Others claim that TAG is a variation of the ontological argument.