I often hear my fellow atheists say things like ‘You can’t prove a negative!’ in a debate. This is often in response to either a theist trying to shift the burden of proof, or a misunderstanding of epistemic justification (aka burden of rationality or burden of proof on a belief position).

I often respond to them, hoping to get them to consider their thoughts, ‘I think what you mean is; you cannot prove non-existence without a doubt’ and often following up with ‘I can prove to you right now that there isn’t a dragon in my living room’.

Absurdisms and moving the goalposts

Typically at this point, rather than take the point, they start responding with absurdisms like ‘I think the dragon is invisible’ ‘I think the dragon is microscopic’ and moving further and further past the standard definition of a dragon. I have learned from this and now specify ‘a 5 ft green visible corporeal dragon’. In fact, some of the responses I get are not even those moving the goal post absurdisms. They are unjust insults, and I find myself again disappointed at the internet atheist community.

Although you can prove some types of non-existence…

However, my original statement is not completely correct either. There are some things you can prove do not exist without a doubt. These are generally definitional or illogical in nature. For example; a square circle, this is impossible. Water that isn’t H2O – sure you might find something that has similar properties to water, but water is defined as H2O. An invisible green monkey – something invisible cannot be green. We could even say the same for god claims that are equally illogical. Many deities cannot exist the way they are described in their holy books. That doesn’t necessarily mean there isn’t one out there, but if that holy book is the evidence that deity exists, and it contains those logical errors, I think it safe to say that that particular piece of evidence does not hold up to scrutiny.

Of course, we would generally regard these things as absurd or incoherent, right? So what about things that are clearly defined and are not illogical.

What about something not logically impossible?

A unicorn, if you take the magical properties out, is essentially a horse with a horn. If we look through the fossil layers we can see no records of this at all. All we have is depictions of mythical creatures. However, we have to understand the fossilisation process is not absolute. Not every creature will have been preserved in this layer, and it is far from perfect. What we can say is that there have been no contemporary sightings of unicorns and that there is no evidence they have ever existed.

Indeed, this is what I was speaking about proving non-existence without a doubt. I cannot prove that unicorns do not exist without a doubt, but with the overwhelming lack of credible convincing evidence, I conclude that they do not.

There are also other ‘negatives’ that can be proven too. A mathematical negative for example… 0 – 1 = -1. Simple, basic, but again is a negative that you can indeed ‘prove’.

So what does that mean for gods?

Well, the first thing is to get the theist to define their particular deity. If there are any logical inconsistencies or they have to rely on fallacies to fit their god into reality then it is fair to conclude that deity does not exist, at least not as they are describing.

Consider if they describe their deity to be a loving deity and then also say their deity committed mass genocide, yet is also all-powerful so could have done it any other way but chose the most violent version possible, and is all-knowing yet get’s disappointed etc – it is safe to say that the deity described is done so incoherently and unless there is some clarification and better evidence then that deity does not exist.

However, if they do have a more simplistic claim, like the deist approach where the deity effectively jump-started the big bang and let cause and effect take care of the rest, there is no way you can really prove this claim either way, but with a deity like this, there is no real difference to there being no deity that started it all and isn’t worth much consideration.

I find the ‘can’t prove a negative’ comment incredibly lazy. I think it has largely come from atheists copying other atheists instead of really thinking about things. We’ve all done it of course, especially when we are younger, most of what we do is imitation. The main frustrating thing I find is a community that prides itself on being so intelligent and rational persists with a number of glaring logical errors.

Folks, come on. You’re better than this… or so you say.

(Check the ‘Bad Atheist Arguments‘ series for more info).

So where do you think this ‘can’t prove a negative’ comes from?

Well, I assume it started with a theist trying to shift the burden of proof with a statement like ‘you can’t prove God doesn’t exist’ and the response is along the lines of ‘how do I test for your God’s existence?’ and other statements like ‘If there is no falsifiable evidence then how exactly can I test for this? You’re essentially asserting that something incorporeal with no way of testing for it exists.’ Perhaps this is what first inspired statement. (If anyone knows where I can find more out on this, please let me know).

Essentially it is more of a case of ‘inability to disprove does not prove it true’ rather than ‘you can’t prove a negative’.

As I have shown above, we can prove some negatives, e.g. I can film my living room, providing evidence of absence for the thing I am claiming is not there. We can prove some things are logically impossible, like square circles. We can prove some things are impossible due to what defines them, like water that is not H2O.

We really need to stop saying ‘you can’t prove a negative’ because you can!

So in conclusion

  • When most say ‘can’t prove a negative’ they are wrong, because you can prove a negative.
  • A more accurate statement ‘you can’t prove non-existence without a doubt’
  • Even then there are some things that you CAN prove the non-existence of, e.g. when they are definitional things or logically incoherent.