There are better and worse reasons for being only a negative atheist. But the one that was argued by my opponent in the discussion was pretty weak — and if it is accepted by others who call themselves atheists, they really should be aware of that.
Briefly, my opponent's argument was that one should only believe when there is evidence; that there is no evidence that there are no gods; and therefore that to positively disbelieve in such beings is completely unjustified.
I suspect some might accept this argument as a result of thinking of evidence exclusively as direct evidence. One can have direct evidence that there are horses by, for example, seeing some. But one cannot have direct evidence that there are no unicorns by seeing none. This, however, ignores indirect evidence. And there is plenty of that regarding unicorns.
One is justified in positively disbelieving in unicorns when one knows certain things about this supposed creature — for instance, that the earliest reports of it were based on long-horned animals depicted in profile (and as a result showing only one horn), as well as on descriptions of rhinos to ancient Greeks who had never seen them; that the spiral tusks of narwhals were sometimes found on beaches and thought to be unicorn horns; that no unicorn or unicorn skeleton has ever been found (which would be very unlikely, given that it is supposedly a large animal); and so on. All this points to the unicorn being a mythical creature — and as that is by far the most likely conclusion, it is reasonable to hold that unicorns aren't real.
Similar kinds of things can be said about gods. There is evidence that gods are human inventions, and even reasonable explanations for why human beings invented them. There are things we know about our existence (e.g., that we are evolved, physical entities) that make the existence of beings who are like us in many respects (e.g., having minds much like ours), yet exist in a supernatural realm or have supernatural powers, extremely unlikely at best. There is the fact that no god, and no act of a god, has ever been observed (there are of course supposed exceptions to this, but the best explanations for them do not require positing any deities) — yet for all of the gods that humans have believed in, this absence is as much of a problem as the absence of unicorns is for unicorn belief. Finally (though of course I wouldn't expect people in general to know this), there is the argument which I made in The Truth about God, that on the traditional meaning of “god”, a god must have libertarian free will — which rules out their existence if libertarian free will is impossible because there is no middle ground between the determined and the random.
Now, one does not have to accept any of these points as conclusive. And one might have other reasons for being a negative atheist. But to claim that positive atheists are mistaken because there is no evidence against the existence of gods is unreasonable.
[Originally published at Debunking Christianity]