Is it true that absence of evidence is not evidence of absence? The best answer is: It depends on the case. Let us consider three cases:
I. There is no evidence for the existence of extraterrestrial life, but the absence of evidence is not evidence that there is no extraterrestrial life. This is because we may have reason to believe that it is improbable for life not to exist somewhere other than Earth.
II. You are investigating a murder case and there are only two persons, A and B, who might have motivation for murdering the victim. A and B are the only suspects; you have some (but not overwhelming) evidence that A committed the crime, but you do not have any evidence that B did it. The absence of evidence that B did it is not evidence that he didn't do it --- he is still a suspect. However, if later on there is more and more evidence that it was A who committed the crime, this will allow you to give more weight to the fact that there is no evidence that B did it, and you can accordingly say that the absence of evidence is now evidence that he didn't do it.
III. Suppose Jennifer believes that there are unicorns. You point out that unicorns are mythical creatures and there is no evidence that they exist. She responds by insisting that the absence of evidence for the existence of unicorns is not evidence that they don't exist. Is this a reasonable response? No, for we have very good reasons for believing that there are no unicorns and there is no need for having any evidence for the truth of the belief.
There can be a more systematic treatment of "Absence of evidence is not evidence of absence"; the point I want to make here is just that we should not accept it as a general principle.
Wai-hung, I agree with your point generally, but I'm not sure that these cases are convincing. In case II, it's not the absence of evidence that is convincing. It's the evidence that A did it, plus, presumably, the evidence that A did not work with B (different motives, don't know each other, etc.). In case III, you mention "very good reasons for believing that there are no unicorns". What is that? Evidence about how the myths were formed? Evidence against the supernatural? It's underdescribed whether any absence of evidence is relevant here.
ReplyDeleteI think a clear case is something like "There is an elephant in the room", where you have very good reason to believe that if there were evidence, you would have acquired it.
Zanja,
ReplyDeleteI didn't say that in case II the absence of evidence itself is convincing. My point was merely that the absence of evidence that B did it can be considered evidence that he didn't do it in the context of the fact that there is strong evidence that A did it.
In my formulation of case III, there is an implicit distinction between evidence and reason --- it is possible for a person to have reason to believe p without having evidence for the truth of p. For example, I have reason to believe that you know your mother's birthday, but I have no evidence that you do.
I tend to think that the lack of evidence for P is never per se evidence that positively supports P. In Case II, intuitions may differ, but I take it that it has to do with the fact that we *presume* innocence in the court of law. In case III, I would take the lack of evidence not to support the unicorn believer. Neither would I take it to support that Unicorn's don't exist. When we say "I don't believe that P", I take it that there are two ways of taking this. The first is as it grammatically appears, with the negation taking wide scope. In other cases, we do mean to express a genuine belief, in this case of not-P.
ReplyDeleteNot sure about this one though.