Socrates was thought by the oracle to be the wisest man because he alone knew how ignorant he was. The knowledge of ignor-ance might have been good at keeping Socrates modest, but it forms a poor basis for deduction. The argumentum ad ignoran-tiam is committed when we use our lack of knowledge about something in order to infer that its opposite is the case.
Ghosts exist all right. Research teams have spent many years and lions of pounds attempting to prove that they don’t; and they have not succeeded.
(The same could probably be said of Aladdin’s lamp and the prospects for world peace.)
The positive version of ad ignorantiam asserts that what has not been disproved must happen. There is, in addition, a negative form which claims that what has not been proved cannot occur.
Talk of extraterrestrial life-forms is nonsense. We know there are none because every single attempt to establish their existence has failed utterly.
(Also true of the Yeti, Bigfoot, the Loch Ness monster and editorial integrity.)
In both versions of the fallacy, the appeal is to ignorance. It is called upon to supply support for an assertion, even though our own state of knowledge does not normally bear on the truth or falsity of that statement. The fallacy consists of the intervention of irrelevant material, in the shape of our own ignorance, into an argument which is about something else. It is notoriously difficult to prove that something exists, especially if it is a shy creature which hides coyly in the deep of a Scottish loch, on the slopes of a mountain wilderness, or in the mists of the third planet of 6 1 – Cygni. You practically have to meet one. Even then, a wealth of recorded evidence would be required to convince others.
To establish non-existence is even more difficult. You have to look at the whole universe simultaneously to make sure that your quarry is not lurking in any part of it. Not surprisingly, this feat is rarely accomplished, and thus leaves us with boundless spaces thickly populated with ad ignorantiams and the other products of our imagination.
Kid, I’ve flown from one side of the galaxy to the other. I’ve seen a lot of strange stuff, but I’ve never seen anything that could make me believe there’s one all-powerful force controlling everything.
Of course there are cases in which our lack of knowledge does influence our judgement; they occur where we would expect to have that knowledge if the thing were true. One would rightly reject a report that Camden Town Hall had been swallowed whole by a slime monster if there were no reports of it in the newspapers, no eyewitness accounts on television, no street celebrations, or any of the evidence we would expect to accompany such an event.
The ad ignorantiam forms the semblance of a cloak to cover the otherwise naked beliefs of those who are predisposed to give credence to extraordinary things. Under its comforting warmth shelters a widespread popular belief in telepathy, poltergeists, demonic possession, magic pyramids, Bermuda triangles and the innocence of tobacco. (Television violence doesn’t do any harm. None of the surveys has ever managed to prove that it does.’)
The argumentum ad ignorantiam is useful if your own views do not follow received opinion. You can persuade others to share these bizarre notions by appealing to the lack of evidence to the contrary. Only slight difficulty is occasioned by the abundance of evidence in many cases to prove you wrong: you reject the evidence, deploying further ad ignorantiams to show that no one has ever proved the evidence to be reliable. In this way you will be able to sustain a preconceived view of things in the teeth of all sense and experience. When you are expert at it, you can add the letters ‘ad ign.’ after those denoting your degree in sociology. After all, no one can prove that you shouldn’t.