Often, when we say that we’re an atheist, what we mean is that we don’t believe in gods. It doesn’t make a lot of sense to respond with “where’s your evidence?”, since it is a negative statement of belief, not a statement of fact. This is what I’ll refer to as weak atheism. Strong atheism on the other hand does concern itself with statements of fact, namely “there are no gods”. Of course, the strong atheist might frame it as “I believe that there are no gods”, but I take that to mean the same as “there are no gods”, since when one expresses that a statement is true they are actually expressing that they believe that the statement is true.
So weak atheism is a state of non-belief and strong atheism is a state of belief. The weak atheist is not making a claim about the world (other than some psychological fact about themselves), and that is why we aren’t concerned with evidence when someone expresses weak atheism, unless we wish to try to cause them to start believing in gods (and this evidence would be in the form of positive arguments for the existence of gods – the only counter-evidence to atheism). The strong atheist on the other hand is making a claim about the world, namely that there are no gods, and must therefore seek to provide evidence for this claim – just as the theist who claims that there is at least one god must likewise provide evidence that it is so.
I believe that there are no gods, and if I voice this belief a theist might justly ask me for evidence. What sort of evidence could I provide?
Firstly, note that I believe that there are no gods, not simply that the god of the Abrahamic religions doesn’t exist. If I provide evidence that there are no gods, it follows that I also automatically provide evidence that the god of the Abrahamic religions doesn’t exist, but this is not the case in reverse. For example, the gods of polytheistic religions might exist, or even if my evidence is strong enough to show that no religion is true, there might be some hitherto unknown god that is believed in by nobody but nevertheless exists. This, it seems to me, rules out the problem of evil, an argument designed to show that there is no all-good god. For even if we show that there is no all-good god, we have still yet to show that there is no somewhat-good god, an indifferent god, an inconsistent god, or even an evil god (although in the case of an all-evil god, one might use an argument roughly symmetrical to the problem of evil to show that this god also doesn’t exist).
Secondly, this evidence need not show that we can be certain that there are no gods. We don’t have absolute certainty that the Battle of Hastings took place in 1066, and yet we can be fully justified in believing that it did based on the historical evidence. So the evidence will be probabilistic, or to put it another way, our proof will be evidential rather than logical.
Thirdly, this evidence will be primarily in the form of philosophical arguments. Scientific data might come into play at certain points (we are talking about the nature of the universe after all), but such data must be accompanied by arguments showing how they work in favour of the proposition that there are no gods.
This is the main question that has been going round my head for the last couple of years or so. It’s a difficult one. I aim to spend a few posts considering the question, and hopefully I can shed some light on what evidence might be brought forward. Any suggestions you might have are welcome, so please feel free to offer them up in the comments!