People prefer certainty. There's a psychological need to explain events or phenomena rather than accept one's ignorance, to say ‘I don't know’ when faced with insufficient evidence.
Imagine you're in a pub and overhear a man talking to a friend about UFOs: ‘Last night I saw a strange red light in the sky, it moved too jerkily to be a plane or helicopter so it must have been an alien spaceship’. This is an argument from personal incredulity (or personal belief or personal conviction), the fallacy of inferring that because you personally find something improbable or hard to believe, it is therefore untrue, and instead your preferred explanation is true.
Even though the man in the pub didn't know what the red light in the sky was, and personally thought it couldn't be an aircraft, that isn't sufficient reason to conclude it's an alien spaceship. At most he should conclude that he doesn't have enough evidence to say what the light was. Most people are actually poor interpreters of aerial phenomena; aircraft and meteorological effects can easily be misconstrued.
Robert Faurisson has written extensively about the Anne Frank diary, claiming it's a fraud perpetrated by Anne's father Otto rather than an authentic account of real events. Much of this claim is an extended argument from personal incredulity. He wrote: ‘The absurdities of the Diary are those of a poor imagination that develops outside of a lived experience. They are worthy of a poor novel or of a poor lie.’ However, what he describes as absurdities are easily explained.
Lack of ability, knowledge or imagination is the crux of the fallacy. Just because one person is unable to explain something that doesn't mean it's unexplainable. Another example of the fallacy is from the debate about evolution and intelligent design. Michael Behe, arguing that irreducibly complex systems must have been designed, wrote:
An irreducibly complex system cannot be produced directly by numerous, successive, slight modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.
Behe's argument rests on his lack of ability to imagine or discover how an irreducibly complex system could have been created by evolutionary steps. In response, several people have shown exactly what Behe was unable to.
The argument from personal incredulity is alluring and superficially convincing because it results from an instinctive need for certainty. It wrongly seeks to shift the burden of proof by drawing on a misplaced confidence in a person's ability to find an explanation for something.
There is a context in which an appeal to personal incredulity may not be fallacious. In law a person is considered innocent until proven guilty. So, for example, it wouldn't be wrong of a juror to find a defendant not guilty if he couldn't determine how the accused had time in which to commit the crime – assuming there isn't independent and sufficient evidence of guilt.
