"Absence of Evidence is not Evidence of Absence"
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[edit] Definition
This maxim is used to illustrate the philosophical difficulties in drawing inferences from negative results. In particular, the failure to find evidence supporting a particular theory or point of view cannot, in general, be considered a demonstration of its falsity.
[edit] Discussion
[edit] An example
Someone might make the statement that "all swans are white." When challenged, he could canvass the local bird-watching community, asking if any of them have ever seen a black swan. When he receives a collection of "no"s, he can triumphantly present this as "proof" of his statement.
The problem, of course, is that the local bird-watchers haven't looked hard enough, or far enough. The Australian black swan (Cygnus atratus) is of course black, but unless you've been to Australia or New Zealand, you probably haven't ever seen one. The local bird-watchers have seen mostly only local birds, an incomplete and unrepresentative sample of all the birds out there.
Similarly, if we have reason to believe that evidence may not be forthcoming or might have been destroyed, then failure to find evidence is not especially convincing. For example, there is no evidence that any species of dinosaurs used wooden tools. However, since dinosaurs lived a minimum of sixty-five million years ago, any wooden tools that they had used would most likely long since have rotted into oblivion, if they ever existed in the first place. To properly address the question "Did dinosaurs use wooden tools" would require a much more sophisticated research protocol than simply looking for artificially sharpened sticks dating back to the Jurassic.
[edit] A counterexample
Conversely, if one has reason to believe that the evidence, if there, would be findable, and that the search has been sufficiently rigorous to find the evidence if it existed at all, it's legitimate to conclude a genuine absence from a lack of evidence. I can demonstrate relatively easily that there is, in fact, no polar bear in my refrigerator. When I look in, I notice a marked absence of evidence for a bear -- and an object the size, shape, and color of a polar bear would be easily visible.
