Copi and Cohen: 93, Davis: 59

By Stephen Downes
http://www.datanation.com/fallacies/distract/ig.htm

Stephen's Guide

Argument from Ignorance
(argumentum ad ignorantiam)

Definition:
Arguments of this form assume that since something has
not been proven false, it is therefore true.

Conversely, such an argument may assume that since
something has not been proven true, it is therefore
false.
(This is a special case of a false dilemma,
since it assumes that all propositions must either be
known to be true or known to be false.)

As Davis writes, "Lack of proof is not proof." (p. 59)

Examples:

(i)   Since you cannot prove that ghosts do not exist, they
     must exist.

(ii)  Since scientists cannot prove that global
     warming will occur, it probably won't.

(iii) Fred said that he is smarter than Jill, but he
     didn't prove it, so it must be false.

Proof:
Identify the proposition in question. Argue that it may be true
even though we don't know whether it is or isn't.

References:
Copi and Cohen: 93, Davis: 59

http://www.datanation.com/fallacies/distract/ig.htm
By Stephen Downes