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Philosophy 203: Scientific Reasoning
The Appeal to Ignorance

Abstract: The argument from ignorance is characterized and shown to be sometimes persuasive but normally fallacious.

I. Argumentum ad Ignorantiam: (appeal to ignorance) the fallacy that a proposition is true simply on the basis that it has not been proved false or that it is false simply because it has not been proved true. This error in reasoning is often expressed with influential rhetoric.
A. The informal structure has two basic patterns:

Statement p is unproved.
Not-p
is true.

Statement not-p is unproved.
p
is true.

B. If one argues that God or telepathy, ghosts, or UFO's do not exist because their existence has not been proven beyond a shadow of doubt, then this fallacy occurs.
C. On the other hand, if one argues that God, telepathy, and so on do exist because their non-existence has not been proved, then one argues fallaciously as well.
II. Some typical ad ignorantiam fallacy examples follow.

In spite of all the talk, not a single flying saucer report has been authenticated. We may assume, therefore, there are not such things as flying saucers.

No one has objected to Lander's parking policies during the last month of classes, so I suppose those policies are very good.

Since the class has no questions concerning the topics discussed in class, the class is ready for a test.

Biology professor to skittish students in lab: There is no evidence that frogs actually feel pain; it is true they exhibit pain behavior, but as they have no consciousness, they feel no pain.

Johnson: It is impractical to send more men to the moon because the money spent for that project could be spent on helping the poor..

Hanson: It is not impractical.

Johnson: Why?

Hanson: Just try to prove that I wrong.
(Hanson is defending his claim by an ad ignorantiam, i.e., his claim is true, if Johnson cannot refute him.)

"The Soviet news agency Tass declared Saturday that the abominable snowman, thought by some to stalk the Himalayan Mountains, does not exist.

Quoting arguments by Vadim Ranov, a man described as a well-known Soviet explorer, Tass said that no remains--skull or individual bones--had ever been found.

Alleged yeti tracks spotted in the mountains are more likely to be those of other animals distorted by bright sunrays, Tass said.

Accounts by 'eye witnesses' are the fruit of their imagination,' the official news agency said." (New York Times)

(Be sure to note why this argument is not a case of the ad verecundiam fallacy.)

"Our universe, however, did begin with the primordial explosion, since we can obtain no information about events that occurred before it. The age of the universe, therefore, is the interval from the big bang to the present." (Scientific American)

III. The uses of the ad ignorantiam in rhetoric and persuasion are often similar to the technique of "raising doubts." E.g., suppose you wanted to convince a police officer not to give you a ticket by using this technique.
"I'm sure you know how unreliable radar detectors are. Why, I saw an a news program a tree was timed at 50 mph, and Florida, at one time, threw out such evidence in court. I certainly wasn't going that fast. Some other driver must have sent back that erroneous signal. You probably timed the car passing me which looked like mine."
IV. Non-fallacious uses of the ad ignorantiam: in science, the law courts, and some specific other situations, one must, for practical reasons, assume that something is false unless it is proved true and vice-versa. E.g., "the assumption of innocence until proved guilty" is a practical, not a logical, process.  Obviously, someone can be legally innocent, but actually guilty of a crime. 
  1. In many instances, if a decision must be made and we cannot prove something in spite of serious attempts to do so, then we presuppose as a pragmatic consideration, without deductive proof,  that whatever that something is, is probably the case.


  2. At one time scientists concluded that DNA would not crystallize because after extensive testing, there was no proof that it would. This conclusion is not fallacious even though now it is known that DNA will crystallize.


  3. There is no fallacy in the following passage:

    "Today we can be confident that a sample of uranium 238, no matter what its origin, will gradually change into lead, and that this transmutation will occur at a rate such that half of the uranium atoms will have become lead in 4.5 billion years. There is no reason to believe that the nature of rate of this process was any different in the very remote past, when the universe was new." Schramm, Scientific American (January, 1974), 67.

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