This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
Things do not exist merely because they have been defined to do so. We
know a lot about the definition of Santa Claus -- what he looks like,
what he does, where he lives, what his reindeer are called, and so on.
But that still doesn't mean that Santa exists.
"Then what if I managed to logically prove that God exists?"
Firstly, before you begin your proof, you must come up with a clear
and precise definition of exactly what you mean by "God". A logical
proof requires a clear definition of that which you are trying to
"But everyone knows what is meant by 'God'!"
Different religions have very different ideas of what 'God' is like;
they even disagree about basic issues such as how many gods there are,
whether they're male or female, and so on. An atheist's idea of what
people mean by the word 'God' may be very different from your own
"OK, so if I define what I mean by 'God', and then logically prove he
exists, will that be enough for you?"
Even after centuries of effort, nobody has come up with a watertight
logical proof of the existence of God. In spite of this, however,
people often feel that they can logically prove that God exists.
Unfortunately, reality is not decided by logic. Even if you could
rigorously prove that God exists, it wouldn't actually get you very
far. It could be that your logical rules do not always preserve truth
-- that your system of logic is flawed. It could be that your premises
are wrong. It could even be that reality is not logically consistent.
In the end, the only way to find out what is really going on is to
observe it. Logic can merely give you an idea where or how to look;
and most logical arguments about God don't even perform that task.
Logic is a useful tool for analyzing data and inferring what is going
on; but if logic and reality disagree, reality wins.
"Then it seems to me that nothing will ever convince you that God
A clear definition of 'God', plus some objective and compelling
supporting evidence, would be enough to convince many atheists.
The evidence must be objective, though; anecdotal evidence of other
people's religious experiences isn't good enough. And strong,
compelling evidence is required, because the existence of God is an
extraordinary claim -- and extraordinary claims require extraordinary