appeal to ignorance

Definition:

A fallacy based on the assumption that a statement must be true if it cannot be proved false.

Examples and Observations:

• "The following two arguments attempt to shift the burden of proof:
  1. There is intelligent life in outer space, for no one has been able to prove that there isn't.
  2. I know that every action we perform is predetermined because no one has proved that we have free will.
  Such fallacious arguments involve an appeal to the emotions in that one hopes to place opponents on the defensive, causing them to believe that the proposed conclusion must be true merely because they cannot prove otherwise. That belief would be irrational, resulting from the feeling of intimidation. In logical argument, it is always the obligation of those who propose conclusions to provide proof."
  (S. Morris Engel, With Good Reason, 3rd ed., St. Martin's Press, 1986)



• "In 1950, when Senator Joseph R. McCarthy (Republican, Wisconsin), was asked about the fortieth name on a list of 81 names of people he claimed were communists working for the United States Department of State, he responded that 'I do not have much information on this except the general statement of the agency that there is nothing in the files to disprove his communist connections.'
  
  "Many of McCarthy's followers took this absence of evidence as proof that the person in question was indeed a communist, a good example of the fallacy of appeal to ignorance. This example also illustrates the importance of not being taken in by this fallacy. No scrap of relevant evidence ever was presented against any of the people charged by Senator McCarthy, yet for several years he enjoyed great popularity and power; his 'witch hunt' ruined many innocent lives."
  (Howard Kahane and Nancy Cavender, Logic and Contemporary Rhetoric, 8th ed., Wadsworth, 1998)

Also Known As: argumentum ad ignorantiam, argument from ignorance