Argument from silence
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The argument from silence (also called argumentum a silentio in Latin) is that the silence of a speaker or writer about X proves or suggests that the speaker or writer is ignorant of X. Whether such an argument is reasonable is subject to some interpretation or debate -- in general, the argument from silence does not offer a rigorous logical proof of a premise, although it may potentially offer some circumstantial evidence for a position.
Here is an easily recognizable example:
- Bobby: I know what sex is.
- Billy: What is it?
- Bobby: I'm not telling you!
- Billy: You're just saying that because you don't know!
Another example of this 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.
Some may consider the use of argument from silence in this situation to be reasonable.
Here is another example using the same argument but in a different context:
- John: Do you know your wife's email 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, in consideration of individual motives.
Scholarly uses of the argument
The argument from silence has also famously been used by skeptics against the virgin birth of Christ. Saint Paul, for example, does not mention the virgin birth, and skeptics therefore argue from his silence that he did not know of it. If this argument is used as an attempted proof of Paul's ignorance, it is a logical fallacy, because ignorance is only one possible reason for Paul's silence: it's also possible that he did not think the virgin birth was important or relevant to his reasoning, or that he referred to it in texts that have now been lost or mutilated. However, the argument from silence is not fallacious if it is used to prove that Paul may have been ignorant. From the fact that Paul refers to the resurrection of Jesus, it is certain that he knew of it. From the fact that Paul does not refer to the virgin birth, it is not certain that he knew of it; therefore, he may have been ignorant of it.
Legal aspects
In some legislative systems juries are explicitly instructed not to infer anything because of an accused person's silence; this is known as the right to silence. For example if an accused said that he was giving a lecture in Spanish at the time a crime was committed, but refused to speak any Spanish, the jury should not conclude that he couldn't speak Spanish and the alibi is false. This in effect bars the use of argument from silence.