Hi Ian,

If the intent of the tool's designers is to mimic human perspectives on
knowledge and logic, then negation as failure is more human like, IMHO, than
any existing alternative.  A person with no experience in an area normally
is very skeptical of assertions that can't be proven within his/her database
of factual and structural knowledge, and reaches the same conclusion.  I'm
sure you've heard it said that you don't know what you don't know, so you
assume you know everything until proven otherwise.

Another way to look at it is that, within the bounds of evidence, a judge or
juror has no basis for any conclusion that is not consistent with known,
demonstrated facts.  It is always possible that other information will
surface in the future, but the rational deduction of the present moment has
to be based on known facts, not on missing information.

One consequence of this result is that it is very hard to convince anyone of
a fact which has no familiarity, in specific or general terms, to them
personally.  That is why attorneys and laws depend on known facts.

-Rich

Sincerely,
Rich Cooper
EnglishLogicKernel.com
Rich AT EnglishLogicKernel DOT com
9 4 9 \ 5 2 5 - 5 7 1 2

-----Original Message-----
From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx

[mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx] On Behalf Of Ian Horrocks
Sent: Monday, August 02, 2010 10:14 AM
To: edbark@xxxxxxxx; [ontolog-forum]
Cc: Bernardo Cuenca Grau
Subject: Re: [ontolog-forum] owl2 and cycL/cycML

It is even more tricky that this. The failure in "negation as failure"
doesn't mean failure of a given algorithm, it means not provably true. There
are many decidable logics with NAF. If we have an incomplete reasoner for
such a logic, we are *still* incorrect if we take failure to return "True"
as being equivalent to "False", because the failure may simply be a symptom
of the incompleteness and nothing to do with NAF.

Simple example: I am using a logic in which negation is interpreted as NAF.
I have a simple boolean theory in which negation isn't used and which
entails A(x). I ask if A(x) is entailed. My incomplete (for entailment)
reasoner answers "False". If I treat this as entailing that A(x) is not
entailed, then I am really incorrect -- nothing to do with NAF.

In fact I think that we would be well advised to strike NAF from the record
-- it's really not helpful in this discussion :-)

Ian





On 2 Aug 2010, at 17:45, Ed Barkmeyer wrote:

>
> Ian Horrocks wrote:
>
>> Regarding my claim that reasoners are typically used in a way that is
actually incorrect, to the best of my knowledge none of the incomplete
reasoners in widespread use in the ontology world even distinguish "false"
from "don't know" -- whatever question you ask, they will return an answer.
Thus, in order to be correct, applications would have to treat *every*
"false" answer as "don't know". I don't know of any application that does
that.
>>
>
> Put another way, it is not incorrect to treat "don't know" as "false",
> if "negation as failure" is a stated principle of the reasoning
> algorithm.  We can state the 'negation as failure' principle generally
> as "if the assertion cannot be proved from the knowledge base, the
> assertion is taken to be false."
>
> Of course, "proved" means that the reasoning algorithm can derive a
> proof, which depends on the algorithm actually implemented in the
> engine.  As Ian mentioned earlier, this kind of "proof" implies that the
> nature of the reasoning algorithm is, or incorporates, "model
> construction", which is typical of various kinds of logic programming
> engines, but there are many hybrid algorithms.
>
> -Ed
>
> --
> Edward J. Barkmeyer                        Email: edbark@xxxxxxxx
> National Institute of Standards & Technology
> Manufacturing Systems Integration Division
> 100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
> Gaithersburg, MD 20899-8263                FAX: +1 301-975-4694
>
> "The opinions expressed above do not reflect consensus of NIST,
> and have not been reviewed by any Government authority."
>
>
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