It is often stated that “absence of evidence is not evidence of absence,” but this maxim is not so clear cut.
Does the fact that we have not found any credible evidence of intelligent life on other planets mean that no such life exists? Here it is true that the absence of evidence is not evidence of absence. The universe is vast, and we have explored only a tiny portion of it. There could be other civilizations millions of light years away. To argue that there could not be complex life on other planets because we have not found it yet is to make an argument from ignorance, a logical fallacy.
And yet, sometimes the absence of evidence is evidence of absence. However, in such cases it may be thought of as positive evidence, rather than an absence of evidence. For example, thoroughly searching your closet and finding no evidence of anyone there is actually positive evidence that there is no one in your closet. The crucial factor is that if there were someone in your closet, you would expect there to be some evidence, such as being able to see them.
Impossible to Prove a Negative?
Of course, it is possible that someone is in your closet, but escaped your detection, perhaps through a method you are not aware of. In fact, it is possible that the world you know does not exist at all, and you are a brain in a jar, with scientists programming a mental world for you, in which there is no one in your closet. As remote as that possibility may seem, it is impossible to disprove it.
It is sometimes said that it is “impossible to prove a negative,” but negative claims are actually affirmative claims about the non-existence of something. It is more useful to think in terms of levels of proof and the burden of proof.
Since it is possible that everything we experience is an illusion, we might say that nothing can be proved beyond all doubt. However, we can prove many things, such as whether or not there is someone in your closet, beyond a reasonable doubt. That is the standard used in criminal courts, and it is sufficient for most people to say that something is true enough to be relied upon.
Russell’s Teapot and the Burden of Proof
There are many things that we cannot prove beyond a reasonable doubt, and we do not have to, because we do not have the burden of proof. The burden of proof is on the person asserting a claim. Bertrand Russell once said that if he were to suggest that there is a china teapot orbiting the Sun between Earth and Mars, undetectable by our most powerful telescopes, the claim could not be disproved. However, if no evidence of the teapot is offered, then we do not need to disprove it.
Asserting the proper burden of proof can be used against many types of superstition and pseudoscience. A person with a strange claim may correctly point out that you cannot disprove what they assert. This does not matter, and to suggest that it does is to commit the logical fallacy of improperly shifting the burden of proof. The burden rests with the person making the claim. If they have no evidence that what they say is true, there is no reason to take their claim seriously.
Silence or Denial as Evidence, Kafkatrapping
When a person is accused of something and they remain silent, could that be evidence of their guilt? Could denying an accusation be evidence of guilt?
Many legal systems recognize the right of criminal defendants to remain silent when accused. Defendants are informed of this right and courts may not make inferences about the person’s guilt based on their silence. However, one reason for this protection is the fact that, logically, silence may indeed be evidence of guilt, if an innocent person might be expected to speak under the circumstances. Some legal systems have introduced loopholes that allow for silence to be used as evidence against a defendant. For instance, the U.S. Supreme Court decided in Salinas v. Texas that a man’s silence could be used against him when police, in a “non-custodial” interview, asked him whether his gun would match shells found at a murder scene, and he did not respond.
It may seem absurd that denying an accusation could be evidence of guilt. However, one can be seen as acting defensive or “protesting too much.” For instance, if someone announces to a group of people that a crime has been committed, and one of them immediately shouts, “It wasn’t me!” it may appear suspicious. Some use the term “kafkatrapping” to describe the fallacious argument that denying an accusation is evidence of guilt, because of the kafkaesque position it puts the accused in. Certain false accusations are so potent that the normal human response of denial could be interpreted as evidence of guilt. Many people answer outrageous claims by saying, “I won’t dignify that with a response.”