Can you prove a negative?

It is often claimed that you cannot prove a negative; that is, you cannot prove that some entity does not exist. Conversely, you can prove a positive by presenting it for observation; for example, you can prove that a rabbit exists by showing your pet rabbit to people.

Consider the assertion ‘Unicorns do not exist’. If someone asserts that unicorns do indeed exist then you can reasonably ask them, ‘Show me one then!’. They will be unable to do so. When you challenge them in their belief, they might reply, ‘Unicorns only appear before people who believe. If you believe first, then they will appear’. You probably will not find this a convincing argument.

Your interlocutor might insist, ‘Unicorns do occasionally leave indirect evidence for non-believers. Consider these hoofprints here’. You point out that those hoofprints look suspiciously like horse hoofprints. Furthermore, you know that there is a horse-riding stable down the road. The reply might be, ‘Yes, they look like horse hoofprints because that’s what the unicorns want you to think. They don’t like making it too obvious. Moreover, look up to the sky: a rainbow has appeared. What more do you want? You can’t prove that unicorns don’t exist; therefore, they exist! The absence of evidence does not mean evidence of absence’. You contemplate saying that rainbows appear naturally etc, but you decide you have better things to do with your time.

In a trivial sense, you can prove at least some negatives. For instance, you can prove that the Moon is not made of cheese because we have been there and collected lunar samples.

The basic epistemological rule of thumb is that the person who asserts the existence of some entity carries the burden of proof; that is, it is up to them to prove positively that something exists, not for you to disprove it.

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