ad ignorantiam
(also known as: appeal to ignorance, appeal to mystery [form of], black swan fallacy [form of], toupee fallacy [form of])
Description: The assumption of a conclusion or fact based primarily on lack of evidence to the contrary. Usually best described by, “absence of evidence is not evidence of absence.”
Logical Forms:
X is true because you cannot prove that X is false.
X is false because you cannot prove that X is true.
Example #1:
Although we have proven that the moon is not made of spare ribs, we have not proven that its core cannot be filled with them; therefore, the moon’s core is filled with spare ribs.
Explanation: There is an infinity of things we cannot prove -- the moon being filled with spare ribs is one of them. Now you might expect that any “reasonable” person would know that the moon can’t be filled with spare ribs, but you would be expecting too much. People make wild claims, and get away with them, simply on the fact that the converse cannot otherwise be proven.
Example #2:
To this very day (at the time of this writing), science has been unable to create life from non-life; therefore, life must be a result of divine intervention.
Explanation: Ignoring the false dilemma, the fact that we have not found a way to create life from non-life is not evidence that there is no way to create life from non-life, nor is it evidence that we will some day be able to; it is just evidence that we do not know how to do it. Confusing ignorance with impossibility (or possibility) is fallacious.
Exception: The assumption of a conclusion or fact deduced from evidence of absence, is not considered a fallacy, but valid reasoning.
Jimbo: Dude, did you spit your gum out in my drink?
Dick: No comment.
Jimbo: (after carefully pouring his drink down the sink looking for gum but finding none...) Jackass!
Tip: Look at all your existing major beliefs and see if they are based more on the lack of evidence than evidence. You might be surprised as to how many actually are.
Variations: The Black Swan Fallacy is committed when one claims, based on past experience, contradictory evidence or claims must be rejected. It is treating the heuristic of induction like an algorithm. The name comes from the claim that “all swans are white” because nobody has ever seen a black swan before... until they did. The reasonable position to hold, assuming you existed in a pre-black-swan world, would be that “all swans that we currently know of are white.” Leave room for discovery unless it has been demonstrated that the contradictory evidence, or claims cannot possibly exist or such claims would be impossible. For example, claiming “all triangles have three sides” is both accurate and reasonable.
The Toupee Fallacy is a cleverly-named variation of the appeal to ignorance where the absence of evidence is the result of the claim made being false. Consider the argument, “all toupées look fake; I've never seen one that I couldn't tell was fake.” The reason the person has never seen one they couldn’t tell was fake is because when they did see one they couldn’t tell was fake, they couldn’t tell was fake. The same goes for penile enlargements and boob jobs.
The Appeal to Mystery is a specific claim stating that the reason we cannot prove something is because “it is a mystery.” Rather than question if the claim is true, we accept that it is true and forego any more investigation by writing it off as a mystery. Why is it that I smell just fine after working out, but everyone else thinks I stink? It’s a mystery!
References:
Walton, D. (2010). Arguments from Ignorance. Penn State Press.
