Personal criticisms and logical fallacies

[D'Arcy J.M. Cain]

On Wed, 10 Feb 2010 01:38:50 +0100
"Alf P. Steinbach" <alfps at start.no> wrote:
> However, although in this particular case the Ad Hominems constituted logical 
> fallacies, not all Ad Hominems are logical fallacies.

Yes they are.  Using the reputation of someone to prove or disprove
their claims is a logical fallacy.

> For example, if a person is a chronic liar, has a known history of lying, then 
> that can have a strong bearing on whether the person's claims  --  technical or 
> about other persons  --  should be seriously considered[1].

Yes but it's still a fallacy.  Taking the author's history into account
may be valid for deciding that further investigation is warranted but by
itself it does not prove anything about the claims.  Suggesting that it
does is fallacious.

"Bill is a liar therefore his statement is false" is a fallacy.  "Bill
is a liar so take his claims with a grain of salt" is not.

There is another case.  "Bill never tells the truth therefore his
claim is wrong" is not an ad hominem fallacy.  It's a sylogism.  It may
or may not be correct but if the first statement is true (Bill always
lies) then the the conclusion is true.

-- 
D'Arcy J.M. Cain <darcy at druid.net>         |  Democracy is three wolves
http://www.druid.net/darcy/                |  and a sheep voting on
+1 416 425 1212     (DoD#0082)    (eNTP)   |  what's for dinner.