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Re: a simple question

From: pat hayes <phayes@ihmc.us>
Date: Thu, 18 Dec 2003 12:19:30 -0800
Message-Id: <p06001f16bc07a98efa53@[]>
To: Drew McDermott <drew.mcdermott@yale.edu>
Cc: www-rdf-rules@w3.org
(Sorry about delay in responding)

I think we are talking at cross purposes and in fact agree on almost 
everything except rhetoric.
    >The NAF approach is likely to be much more efficient, much easier to
    >implement, and much more likely to yield a useful conclusion than the
    >heavy-duty theorem prover.

    [Pat Hayes]
    All true. It is also likely to be wrong,
    unfortunately. The fact that you can't think of a
    closer airport doesn't usually qualify as a good
    reason to conclude that there isn't one, unless
    you also know for sure that you know all the
    airport locations, so that if you don't know it,
    then its not there. Like, for example, if you
    have a  list of all the airports. If you make
    this explicit, as you should, then you are back
    doing 'heavy-duty' reasoning.

I was trying to stay within the vocabulary of the example, and I was
assuming a plausible context that I didn't state, namely that someone
was planning a trip.

And you thereby illustrate my point. When things are published on the 
SWeb, they immediately LOSE their assumed context.  Rules written 
assuming a particular context tend to fail, potentially disastrously, 
when used out of that context. NAF is a particularly acute example of 
this, which fails totally under even a slight change in context.

   If you replace "nearest airport" by "nearest
airport reasonable to travel someplace from here," then negation as
failure is a reasonable strategy, assuming you know all the airports
in the vicinity.

Of course it is a reasonable strategy to USE, particularly if make 
that assumption explicit. In fact if you make it explicit enough, 
then NAF ceases to be 'non-deductive' (or whatever other silly label 
you want to attach to it) and becomes a perfectly valid monotonic 
inference.  But in any case Im not arguing that NAF should not be 
used, when the user knows what they are doing, as a fast heuristic 
method (or even non-heuristic, if used properly) . I am saying that 
as a general inferential strategy it is a very, very bad idea to rely 
on it, particularly applied outside its original context.

There is a fascinating literature in applied psychology on efficient 
inference strategies which rely on 'lack of knowledge' to draw 
conclusions, which generalize NAF in fact. (Girgenzer calls it 'the 
recognition heuristic') So yes, of course this kind of reasoning is 
useful, because efficient. But saying it is USEFUL is not to say that 
it should be assumed as a semantic basis for information exchange on 
the semantic web.

This confuses two issues: strategies for useful reasoning are one 
thing, justifications of conclusions are another. We need both, but 
we need to keep their roles clearly distinguished. To point out that 
NAF is not a good foundation for truth-justification in general is 
not to say that all SW reasoning must be done by clunky 
general-purpose inference engines.

You know, Drew, it is slightly irresponsible of you to be airing 
these old debates in such a forum at this stage in history, IMO.  We 
have had this battle in AI/KR, and surely we have done it to death 
and now all understand these matters reasonably well.  If we re-open 
the procedural/assertional debate now, particularly using the old 
question-begging terminology of mutual recrimination (neat/scruffy, 
proceduralist/logicist, etc.) we will NEVER get any useful work done. 


    BTW, calling it 'heavy-duty' is misleading. In
    the first case you have made all the equality
    reasoning explicit. In a prolog-style
    implementation this is all buried in the
    backtracking done by the interpreter: but it
    still needs to be done. The same actual
    *reasoning* is involved in both cases.

Yes.  But the NAF version is stylized in a way that permits efficient


Permits?? Are you implying that the use of a monotonic logic somehow 
*forbids* efficient implementation ??

There is a deep-seated fallacy surfacing here, to the effect that the 
use of logic (or indeed anything else, but it seems to be usually 
invoked by the use of the L-word) as a representational language 
*requires* that a certain kind of mechanism be used to process it. If 
you use logic for KR, you are *obligated* to use a general-purpose 
complete logical reasoning engine, for example: or if you say 'equal' 
then you must use a GPCLRE which draws conclusions using 
paramodulation, or whatever. This is nonsense.  You can 'do' equality 
reasoning by iterating along a list if you like. It will be 
incomplete, of course, but most efficient reasoners are incomplete: 
so what? Nobody is saying that the use of a logical KR language 
requires all reasoners to be complete. It is logically sound to just 
not draw any conclusions at all, for one thing.

(This confusion between KR language and process is so ubiquitous that 
it deserves a name: how about the "MIT-Yale fallacy" ?  Ah, but that 
would be an unworthy suggestion, reminiscent of the bad old days when 
people swore vengeance on the bodies of their rejected conference 


If you could be sure that the alternative always
involved iterating through a list and doing a set of equality
substitutions, you could probably find an equally efficient
implementation.  (I've often wondered why no one has worked on this.)
In the general case, though, you have to have a system that does
general-purpose reasoning about equality, which can involve a lot of
You do not HAVE to have any kind of system. At some level, all 
reasoning about equality is "general-purpose": after all, equality is 
a pretty generic kind of thing to reason about. What I think you mean 
here, though, is not the kind of *reasoning*, but the kind of 
reasoning *process* or *strategy* that must be used: and then what 
you say is just flat wrong. A reasoner is not in any way OBLIGATED to 
use a complete inference method to handle equality. In fact a 
reasoner is not even obligated to use a valid or guaranteed correct 
inference method. It might for example cut corners by assuming names 
are unique. its conclusions will not be valid, in general, but 
nothing in the semantic specification of the language requires that 
all reasoners only perform valid inferences. The spec only guarantees 
that IF you conform then your conclusions will be as sound as your 

    >  I hope the people who deprecate it realize
    >that the heavy-duty theorem prover is the only alternative.

I should have pointed out more forcibly how totally false this is, 
like a Microsoft salesman saying that only alternative to Word is 
pencil and paper. And the use of 'heavy-duty' is a rhetorical 
flourish that hardly bears close examination. Some of the 
heaviest-duty software ever written spends its time doing database 
NAF-style reasoning.

    Its not a matter of alternatives. If you want to
    draw checkable valid conclusions, then you need
    to do this kind of reasoning. 

I don't want to draw checkable valid conclusions.
Fine: then do whatever you wish in your domain of application. But 
when PUBLISHING your rules, I think it is not unreasonable to have a 
global requirement (or at any rate a code of good practice) that 
whatever you publish, you are responsible for saying what it means 
clearly enough for others to use it. If you publish rules that only 
work in an unstated context and which fail elsewhere, without any 
indication that this is true, then you are acting at best 
irresponsibly; and I would like the overall SW specs to say that you 
are acting in way that fails to conform and is deprecated.

    If you want to
    make random guesses and hope for the best then
    you can of course work faster, but don't expect
    others to believe in your conclusions.

At least I'll _have_ conclusions.
The rhetorical point being that any poor fool who trusts to a clunky 
FO theorem-prover won't get any in a single lifetime, right? Drew, 
where have YOU been?? Moore's law and about 20 years of dedicated 
hacking better unifiers, etc., has made even general-purpose 
reasoners quite able to handle a lot of useful cases. Not nearly to 
the scale obtainable with DLs or database technology, of course, but 
still of some utility.

    Negation-as-failure is NOT a good general
    reasoning strategy: 99.99% of the time it will
    immediately produce childishly ludicrous
    conclusions: I don't know anyone called Jose, so
    there isn't anyone called Jose; I never heard of
    SARS, ...

Where have you been? 

Of course negation-as-failure is not the way to handle "not" in
general; it's the way to handle it when you don't care about possible
nearby secret airports and the like.
and when you have some reason to suppose that airports that you care 
about are known to you. Fine: so make this assumption explicit 
somehow, and then NAF as an efficient inference *method* is freely 
available for exchange, since *once that assumption is made 
explicit*, NAF is monotonic (and hence a perfectly good form of 
'syllogism', to use the ignorant terminology which started this 

    industrial uses of Prolog-style rules all are
    designed within controlled environments,
    typically using databases, where such special
    conditions can be assumed.

To repeat what I said above, if you use NAF as an efficient way to
draw valid conclusions, you're right.  I prefer to think of it as a
way to draw conclusions that may well be wrong, in situations where
the wrongness of a probably correct conclusion is not fatal.  The
burden is on someone who finds this distasteful to show that pure
deductive techniques will suffice for real-world applications.

No,  it has got nothing to do with showing anything about techniques. 
People should, and will, use whatever techniques they find useful, 
and good luck to them.  None of the SW specs (RDF, RDFS, OWL) say 
anything about what  techniques can or must be used to process these 
languages (except for owl:imports). 

The burden is to show how conclusions generated in this way can be 
published without misleading someone  who is unaware of the context 
in which they were derived, and to provide for ways of publishing the 
rules themselves so that their assumed preconditions of use can be 
made clear.  Several ways have been suggested, including having a 
distinct 'failure-negation' . My own favorite is to have a notation 
for saying explicitly that some ontology is a closed world as far as 
a namespace is concerned, and then NAF is just plain valid when 
applied properly,and NAF and logicism can coexist on the Web happily. 


                                              -- Drew

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Received on Thursday, 18 December 2003 15:36:24 UTC

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