Monday, May 22, 2006

Self-Deprecating Testimony

Suppose someone tells you that they're an unreliable source. Should you believe them?

This isn't so bad as the liar paradox. The statement might be true, after all. Or it might just as well be false.

If they are a reliable source (but simply misleading you in this particular case) then the statement is false, so you shouldn't believe it. On the other hand, if they're an unreliable source (but having a rare moment of honesty) then the statement is true, so you should believe it. Oddly enough, the statement itself doesn't seem to provide you with any information at all about which of these cases is more likely.* All it tells you is that the person is certainly not perfectly reliable, nor perfectly unreliable (in the sense of reliably telling only falsehoods).

* = You might think we should have a default disposition towards believing others. But that might just as well be described as a default assumption that their testimony is reliable -- which should lead us to disbelieve them in the present case. Actually, that sounds right. If someone said to me "most things I say are false", I would think it more likely that that particular thing they just said was false. So perhaps we should simply disregard such self-deprecating testimony?



  1. 1) How many reliable people are going to tell you that they are unreliable - I would say a low fraction.
    2) How many unreliable people are going to tell you they are unreliable - I'd say a much higher fraction (even if still a small one)
    3) how many unreliable people are there in the world relative to reliable people (function of your definition I guess)

    Result can be used to calculate your belief.
    I suggest they are probably unreliable but I am very open to being disproved since this required quite a lot of assumptions on my behalf and an opinion on a definition.

  2. Equating an 'unreliable source' to a liar is a strange way to use the word. A 'source' is usually on some topic, not every topic. There are no general 'reliable sources' on every topic, unless you believe in omniscience.

    So people can perfectly truthfully say they are an unreliable source *on X topic*. Unless that topic is themself, then you get your quandary. But the answer to your question about what the truth value of their statement is in that case is pretty simple: Unknown.


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