Argument from silence

The argument from silence (also called argumentum ex silentio in Latin) is generally a conclusion based on silence or lack of contrary evidence. In the field of classical studies, it often refers to the deduction from the lack of references to a subject in the available writings of an author to the conclusion that he was ignorant of it. When used as a logical proof in pure reasoning, the argument is classed among the fallacies, but an argument from silence can be a convincing form of abductive reasoning.

Textbook examples
Here is an easily recognizable example:
 * Bobby: I know where Mary lives.
 * Billy: Where?
 * Bobby: I'm not telling you!
 * Billy: You're just saying that because you don't know!

Billy's conclusion may not be justified: perhaps Bobby doesn't want to tell him. Consider, however, the following type of argument:


 * John: Do you know any Spanish?
 * Jack: Of course. I speak it like a native.
 * John: That's good, because I need to know the Spanish phrase for "Happy Birthday".
 * Jack: Sorry, I don't have time for that right now. Maybe tomorrow. Bye.

Afterwards, Jack continually refuses to give John the Spanish translation, either by ignoring John or by giving excuses. John then concludes, by argument from silence, that Jack does not in fact know Spanish or does not know it well. In other words, John believes that Jack's ignorance is the most plausible explanation for his silence. Use of argument from silence in this situation is reasonable given that the alternatives, that Jack either doesn't want or is afraid to translate, would be unreasonable without more information.

Here is another example using the same argument but in a different context:
 * John: Do you know your wife's e-mail password?
 * Jack: Yes, I do as a matter of fact.
 * John: What is it?
 * Jack: Hey, that's none of your business.

When John repeatedly asked for the password, Jack ignores him completely. Thus, using the argument from silence, John concludes that Jack does not actually know the password. Such an argument from silence, in contrast, may be considered unreasonable, since a password is a security feature not intended to be shared with a stranger simply because they asked. It may be reasonable, by contrast, to assume that Jack does indeed know the password but refuses to say it for legitimate security concerns.

Scholarly uses of the argument
The argument from silence is convincing when mentioning a fact can be seen as so natural that its omission is a good reason to assume ignorance. For example, while the editors of Yerushalmi and Bavli mention the other community, most scholars believe these documents were written independently. Louis Jacobs writes, "If the editors of either had had access to an actual text of the other, it is inconceivable that they would not have mentioned this. Here the argument from silence is very convincing."

In the Nova/Horizon episode "The Case of the Bermuda Triangle", David Kusche pointed out that the argument from silence has been used to create an improper shifting of the burden of proof in many Bermuda Triangle stories and theories:

"Say I claim that a parrot has been kidnapped to teach aliens human language and I challenge you to prove that is not true. You can even use Einstein's Theory of Relativity if you like. There is simply no way to prove such a claim untrue. The burden of proof should be on the people who make these statements, to show where they got their information from, to see if their conclusions and interpretations are valid, and if they have left anything out."

Legal aspects
In some legal systems (for example, the German one) juries are explicitly instructed not to infer anything because of an accused person's silence; this is known as the right to silence. Thus, the jury may not infer anything from the accused's failure to testify. This in effect bars the use of argument from silence.

On the other hand, statements volunteered by the accused may normally be considered, and in such cases the argument from silence may apply in a limited form. If the accused chooses to testify, the right to silence is forfeited as regards that proceeding. Witnesses also normally have a right to silence as regards any question that is factually incriminating, but that right only bars the jury from making inferences about the witness's conduct. The range of inferences available about the defendant's conduct will vary.

Some versions of the Hearsay Rule use a form of argument by silence in that if a party fails to respond to a statement that would normally provoke a denial, this statement is considered as having been adopted by that party and can generally be used against him or her at trial. The reasoning behind this is that if the party had disagreed with the statement, he or she would have objected.

Notes and reference
Argument per silenci Argumento ad silentio Argumentum a silentio Argomento del silenzio אקס סילנטיו Argumentas iš tylėjimo Argumentum ex silentio