This is well-recognised as an unfair attempt to shift the burden of proof, rather than an assertion that what I allege is true, because we don't know that it isn't.
It may also be considered a legitimate response to a disputant who refuses to put forward any proposal in a situation where immediate action is required.
A defeasible argument
Most interestingly, the appeal to ignorance can be treated as an example of defeasible, presumptive reasoning.
In such instances, the lack of evidence for a proposition can be seen as supporting a reasonable belief that it is not, in fact the case.
This is the basis for the proverb 'no news is good news' – the lack of news of a bad event is taken as reason to believe that nothing bad has actually happened.
Arguments from ignorance are of great importance to Walton as they are the archetypal form of presumptive reasoning, a form of inference which always operates on an incomplete set of data.
He has 4 related schemes, the main argument from ignorance, two sub-schemes from negative reasoning and an epistemic argument from ignorance. This last is a finer-grained extrapolation from the first rather than a new form of argument.
Major premise: If A were true, then A would be known to be true.
Minor premise: It is not the case that A is known to be true.
Conclusion: Therefore, A is not true.
This version is largely unobjectionable, save for the use of 'would be known' which invites insoluable disputes about what is known, by whom and on what grounds. Imagine an atheist arguing that God is not known to exist.
Negative practical reasoning
This is placed as a sub-scheme. It is, I think, a very different argument.
P1: I do not know whether A is true or not.
P2: I have to act on the presumption that A is true or not true.
P3: If I act on the presumption that A is true, and A is not true, consequences B will follow.
P4: If I act on the presumption that A is not true, and A is true, consequences C will follow.
P5: Consequences B(C) are more serious than consequences C(B).
Conclusion: Therefore, I act on the presumption that A is not true (true).
P2. I have to do something with this gun, acting as if it were loaded or as if it were not.
P3. If I act on the presumption that it is, but in fact it isn't, I waste some time.
P4. If I act on the presumption that it isn't, but in fact it is, someone might get shot.
P5. Someone's being shot is more serious than my wasting my time.
Conclusion: I should act as though the gun were loaded.
Which is fine. It shows in the form of a defeasible scheme how the precautionary principle of 'better safe than sorry' works in practice.
Wagemans (2003) expresses this in terms of two concepts of reasonableness: the geometrical and the anthropological, where the former is related to logical connections and the latter to community norms.
But is it an argument from ignorance?
I suggest 3 reasons why it is not:
1. The argument does not give us a reason to believe that the gun is loaded – only a reason to behave as if it were.
2. The chances of the gun's being loaded may be tiny, a million to one, but the principle still applies. That is not the case for arguments from ignorance.
3. The argument rests entirely on our prediction of the consequences of an action. It should be categorised as an argument from consequences.
Negative reasoning from normal expectations
Major premise: If the situation were normal, A would be true.
Minor premise: It is not the case that A is true.
Conclusion: Therefore, the situation is not normal.
It is hard to see in what way this argument is based upon ignorance. If we know that A is not true, then where is the ignorance? Indeed, Walton's own example doesn't fit well here: it's a classic argument from ignorance using expectations – nothing to do with what is 'normal'.
A: Is so-and-so still in jail?
B: Probably, because we would have heard if she had been released.
So, Walton's expansions of the basic form are unhelpful in our understanding of appeals to ignorance as they feature very different arguments.
Kreider's odd contribution
A.J. Kreider has described arguments from ignorance as 'abductive inferences'. That is, inferences to the best explanation. He dismisses the view that they are enthymatic as this presupposes too much about the arguer. And:
I fail to see why this is troubling. Kreider fails to see that his scheme doesn't work without the enthymatic premise.
Abduction is usually defined as inference to the best explanation. There will, however, always be a reason why one explanation is better than the others. His example:
What is the best explanation for the fact that there is no evidence of a tiger in the room? It is, of course, that there is no tiger in the room.
But why is this a good explanation? Because we expect tigers to leave evidence?
P1: There's no evidence of a tiger in the room.
Conclusion: There's no tiger in the room.
This only makes sense if we add:
(P2): If there were a tiger in the room, there would be evidence.
Since abductive reasoning is still reasoning – not just snatching conclusions from the air.
A lack of evidence does not have to mean ignorance. If a data set is known to be complete, then a lack of something within it is evidence of that thing's non-existence.
The lack of Poland on a complete list of world cup winners is not a lack of evidence of Poland's having won. Arguing from such a lack to the conclusion that Poland have never won the world cup, is a deduction, not an argument from ignorance!
P1. There is no reliable evidence available to us of p.
P2. It is reasonable to expect that if p were true, there would be reliable evidence available to us of p.
Therefore: It is reasonable to state (believe) that p is not true.
I don't see a problem with crediting arguers with P2 even when it is unstated as the inference of the conclusion from P1 seems to imply it. The problem of 'being known' from Walton is avoided, and replaced with the more relevant 'reliable evidence available' which leads to obvious critical questions.
A significnt lack of evidence
So, a lack of evidence is significant when it would be reasonably be expected.
The Iraqi Weapons of Mass Destruction saga illustrates this type of reasoning.
The allies were desperate to find evidence of WMDs in Iraq precisely because they knew such weapons were likely to leave evidence and any lack of that evidence would be significant. Having failed to find any evidence, all the signifcant figures involved have admitted that there were none.
To the embarassment of some...
Kreider, A. (2016) Informal fallacies as abductive inferences. Logic And logical philosophy, Volume 25, 73・82.
Locke, J. (1690/1975). An Essay Concerning Human Understanding. Oxford: Oxford University Press.
Wagemans, J. (2003) Conceptualizing fallacies: The informal logic and pragma-dialectic approaches to the argumentum ad ignorantiam. In: Proceedings of the fifth conference of the International Society for the Study of Argumentation. F.H. van Eemeren, J.A. Blair, Ch.A. Willard & A.F. Snoeck Henkemans (eds). Amsterdam: Sic Sat
Walton, D. (1992) Nonfallacious arguments from ignorance. American philosophical quarterly, volume 29, number 4, 381-387.
Walton, D. (1996) Arguments from Ignorance. University Park, Pennsylvania: Penn State Press.
Walton, D, Reed, C. & F. Macagno (2008) Argumentation Schemes. Cambridge: CUP.