CuSithBell comments on Undiscriminating Skepticism - Less Wrong

97 Post author: Eliezer_Yudkowsky 14 March 2010 11:23PM

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Comment author: wedrifid 22 January 2012 06:34:15PM *  0 points [-]

Jayson_Virissimo is talking about logical validity.

Yes, we both are. We have gone as far as to accept a shared definition of logical validity and trace the dispute from there.

The argument is not logically valid, because it is possible for "Z is likely to be correct" to be false, even if the other statements are true (for instance, add the premise "Z is incorrect").

This is simply false. The following premise:

Things said about Y by a person who has a reputation for being an expert on Y are likely to be correct.

... becomes invalid the moment there is in fact a "thing said <etc, etc>" that is not likely to be correct. That's why I put it there! It is an instance of the class of premise "ALL G ARE W" and so just like all other premises in that class it is false if there is a G that is NOT W. it just so happens that 'likelyhood' is the subject matter here.

The above serves to make the premise in question rather brittle. While it does means that the whole argument can be treated as deductive reasoning (about the subject of likelyhoods) it is also means that there are very few worlds for which that premise is true and meaningful.

Comment author: CuSithBell 22 January 2012 08:11:53PM 0 points [-]

I interpreted your premise as: (Things said about Y by a person who has a reputation for being an expert on Y) are likely to be (correct.) as opposed to (Things said about Y by a person who has a reputation for being an expert on Y) are (likely to be correct.)

If, as you seem to be agreeing, a thing cannot be "likely to be correct" and "incorrect" (as known by the same reasoner), then the premise reduces to "Things said about Y by a person who has a reputation for being an expert on Y are correct".

Is this really what you intended?

Comment author: wedrifid 22 January 2012 09:25:30PM -1 points [-]

I interpreted your premise as: (Things said about Y by a person who has a reputation for being an expert on Y) are likely to be (correct.) as opposed to (Things said about Y by a person who has a reputation for being an expert on Y) are (likely to be correct.)

The second was the intended meaning.

If, as you seem to be agreeing, a thing cannot be "likely to be correct" and "incorrect" (as known by the same reasoner), then the premise reduces to "Things said about Y by a person who has a reputation for being an expert on Y are correct".

Given the 'as known by the same reasoner' clause wouldn't that imply that it is '<...> cannot be known to be incorrect'? Either way it is clear that the encapsulation of the probabilistic parts is woefully inadequate here.

Is this really what you intended?

No, but it does seem to be the implication.