An argument from ignorance attempts to argue that something must be true because it has not been definitively shown to be false. Such reasoning is fallacious because it is possible, and in certain circumstances highly likely, that a statement may be false even though it has not been proven to be false, or true even if it has not yet been proved to be true. Examples of an argument from ignorance include:
“Aliens have almost certainly visited Earth; after all the universe is so vast and we know so little about what’s out there, and no one has ever proved that aliens couldn’t be visiting us.”
“You should continue to take these drugs. After all, no one has ever shown them to be ineffective.”
“I’ve never heard about Bob doing anything wrong, so I suppose he must be a nice guy.”
Arguments from ignorance confuse the two concepts of ‘absence of evidence’ (i.e. the lack of evidence one way or the other), and ‘evidence of absence’ (i.e. evidence that something did not happen or does not exist). No conclusions can be drawn from an absence of evidence – all that can be said is that we have insufficient information to make any firm judgment. Evidence of absence, however, does allow us to draw the conclusion that the event or object in question is unlikely to have occurred or to exist.
It can sometimes be difficult to judge whether a given inference is justified, or whether it constitutes an argument from ignorance. In such cases, the fundamental question to consider is whether I would expect to have seen evidence of the event or thing in question if it occurred or existed. If not, then there is an absence of evidence and no conclusions can be drawn. Conversely, if we would reasonably have expected to see such evidence, then there is evidence of absence and conclusions can be drawn. For example, there is no reason why I should expect to see any evidence of aliens living on very distant planets, and therefore on this matter we have an absence of evidence. On the other hand, if I were to consult a train timetable and fail to see any train listed as departing at 4:30pm, it is reasonable to conclude that no such train exists, for if a train did depart at that time then I would reasonably expect to see it on the timetable.
Arguing from ignorance: overview from Logical Fallacies
Argument from ignorance: overview from Logically Fallacious
Argument to ignorance: overview from Skeptic’s Dictionary
Argumentum ad ignorantiam: more extended treatment with examples of genuine and false uses