Ad hoc fallacy

In July 2009, Danish psychic/dowser Connie Sonne was given the chance to prove her claimed dowsing ability in the One Million Dollar Paranormal Challenge offered by the James Randi Educational Foundation. She was asked to dowse some randomly selected cards hidden in envelopes and lost the challenge by selecting other incorrect ones. In an interview afterward, she insisted that she lost merely because, “…it wasn’t time yet for my powers to be revealed.”

The ad hoc fallacy is not strictly an error of logic. Instead, it a fallacious rhetorical tactic in which a person presents a new explanation that is unjustified or simply unreasonable, in an attempt to rescue their original claim after evidence that contradicts it has emerged.

The Latin phrase “ad hoc” is literally translated as meaning “to this”. It refers to an idea or solution that is intended for a specific use, and not for any other uses. An ad hoc explanation is specifically constructed to be used in a particular case and is created hastily at the moment rather than being the result of deliberate, fact-based reasoning.

Another example encountered by skeptical investigators is as follows. This is a typical conversation between a supposed psychic who claims to be able to read minds and a skeptic.

Skeptic: “If you’re psychic then tell me what number I am thinking of”                        

Psychic: “My powers don’t work in the presence of skeptics.”

In this example, the fallacious tactic is pretty obvious. The response that their powers don’t work around skeptics is clearly a ridiculous explanation, and it’s an explanation that one would never accept unless one was already convinced that the person was a psychic. Further, it makes it impossible to discredit them no matter how fraudulent they actually are (a lack of falsifiability is a hallmark of ad hoc fallacies).

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