Argumentum ad ignorantiam

This fallacy is committed when a claim is believed to be true because it has not been proved false, or vice versa.

The general form is:

1. A states that claim X is true.
2. B states that claim X is not accepted as true, due to lack of evidence.
3. A states that claim X is potentially true despite the lack of evidence because B has not proved it false (wrongly shifting the burden of proof to B).

Examples:

• You can't say that PSI phenomena don't exist; after all, science hasn't disproved them.
• We have no knowledge of alien visitors; therefore, aliens do not exist. (They may well exist, but are not visiting Earth).
• Just because Reiki healing energy cannot be detected does not mean that it isn't there.

The defining feature of the Argument to Ignorance is that ignorance, the lack of knowledge, is used as evidence to support/refute a claim whereas in reality, it's an irrelevance. Claims can only be accepted or refuted based on evidence. Ignorance about something provides no evidence about its existence or non-existence.

The burden of proof is a concept where the claimant must provide proof or evidence of the claim they are making before it can be considered. It's not up to others to refute a claim that has no evidence to support it. This position is a logical one, which is meant to avoid false conclusions. In UK law, for example, a defendant is assumed innocent until proved guilty. It is up to the accuser (or the prosecution) to provide evidence for their claim.

Contradictory claims.

Contradictory claims arise from dichotomous (either/or) situations. The claim can only be true or false, therefore proving one proposition to be true or false settles the claim. For example, Person X is either male or female. This person's gender (let's assume female) can be discovered either with evidence that she's female or evidence that she's not male.

Contrary claims.

Contrary claims are similar to contradictory ones in that only one proposition can be true; however, they differ in that both, or more, propositions can be false: i.e. proving one proposition to be false does not automatically prove another to be true.

Example: Person X drives a German car. Audi and BMW are popular German cars. Person X does not drive an Audi therefore Person X drives a BMW (this is clearly not necessarily true).

Note: when in our state of ignorance with both types of claim, our ignorance cannot be used to argue for or against the truth of the claim.

The argument to ignorance - general usage.

The argument to ignorance is used extensively. It is mainly used in two ways:

1. As an argumentative tactic.
   
   It is rare to engage in debate about controversial or unaccepted topics without encountering an argument to ignorance. "No-one's proved it doesn't work" and "just because you can't see it doesn't mean it isn't real" are common examples. The example of Person X illustrates the fallacy: "no-one's proved Person X isn't a man" does not add any weight to the claim that Person X is a man.
   
   Similarly, "no-one's proved that ghosts don't exist" adds no weight to claims that they do.
   
   
2. As a proposition in claims.
   
   Most claims we deal with are contrary claims rather than contradictory claims. This means that showing a proposition in a claim to be false does not mean that an alternative proposition is true. Many people, however, treat claims as if they were contradictory: creating a false dichotomy.
   
   A ghost hunter may get an unusual reading on an EMF meter and postulate that it may be due to ghost activity. A skeptic may point out that it may have come from electric cables. If the ghost hunter discovers that there's no electricity in the building, thus disproving this explanation, he may feel justified in his conclusion that he's picked up a ghost. This is obviously a false dichotomy fallacy; however, it is still based on an argument to ignorance: the idea that not knowing the cause of the unusual reading adds weight to the claim that a ghost was encountered.
   
   It is important to realise that someone showing that a proposition in an argument made against them is false can still leave them making an argument to ignorance. The example of the German cars should illustrate this point.

When is ignorance not a fallacy?

The Argument to Ignorance fallacy is committed when a claim is believed to be true because it hasn't been proved false, or vice versa. Whenever one points out that a person is using the Argument to Ignorance fallacy, such as believing in something because it hasn't been proved false, it is common to get a reply along the lines that it works both ways: one can't state that something is false just because it hasn't been proved true either!

This can of course be true but it is not always the case:

We have to consider the burden of proof requirement. If a person makes a claim it is up to that person to provide supporting evidence. If there is none it is not committing an Argument to Ignorance to assume that a claim is false until some supporting evidence is provided.

"Absence of evidence is not evidence of absence" is a saying that's often used. It is true in some circumstances (in the possibility of there being alien life, for example) but not all. It is not true when there's an absence of evidence and we should expect to find that evidence. For example, stating that the Loch Ness Monster could exist because no-one's proved it doesn't is clearly an Argument to Ignorance; however, stating that the Loch Ness Monster doesn't exist because no-one has proved that it does is not making an Argument to Ignorance. This is because a breeding population of Plesiosaurs (or whatever) would be expected to leave evidence of their existence.

Conclusion.

Arguments from Ignorance are extremely common and can be easy to miss. They can be obvious when formally stated as "I believe X and science hasn't proved X wrong"; however, they often take the rather more persuasive form: "I believe that X is true. It may not certainly be true but science has never shown X to be wrong therefore it's reasonable to keep an open mind on X".

The defining features are:

1. A case is put forward that has no tangible supporting evidence;
2. It's stated that the claim has not been proved false therefore it is reasonable to believe it;
3. The burden of proof will be shifted from the claimant to the opponent - challenging the opponent to prove it wrong;
4. As the name implies, it is ignorance itself, the lack of knowledge of the claim (and by definition no possibility of a counter-claim), that is used to support it.

Claims can only be decided based on evidence. If something is not real or doesn't exist then no evidence can be found that will support or refute it. To claim that something has never been disproved merely adds weight to the fact that it probably isn't true as no evidence exists which could be used to examine it; and as always, it's the quality of evidence that decides a claim - not the lack of it.