Negative proof

"No one has produced an example of one; therefore it doesn't exist."

While the assertion has some strength as an epistemic directive, logically it has none. Logically one may argue that just because we have not observed a particular phenomenon, that is no reason to exclude its possibility. The epistemological slant though is to require a demonstration, before one admits that one is compelled to admit the phenomenon as real.

The problem with the argument is precisely that it has no logical force, but is nevertheless a necessary guiding principle for scientists.

On the converse someone disputing the ancestry of man amongst the lower apes; will invariably point to the lack of demonstrated intermediary stages (The missing link[?]). The often used rejoinder to this and other similar proffered arguments is: "Absence of evidence is not evidence of absence!"

A proper resolution of these conflicting uses is a delicate task.

See also: Argument from silence; Lack of imagination