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Re: Impact of monotonicity in RDF (was: Social Meaning and RDF)

From: pat hayes <phayes@ai.uwf.edu>
Date: Thu, 6 Feb 2003 17:12:30 -0600
Message-Id: <p05111b11ba6898c66024@[10.0.100.86]>
To: Seth Russell <seth@robustai.net>
Cc: <www-rdf-interest@w3.org>, Graham Klyne <GK@ninebynine.org>

>Graham Klyne wrote:
>>.........
>>
>>Above, I've tried to answer your question "could you elaborate on 
>>what this restraint really entails" (or maybe what it does not 
>>entail?).  One has to choose the meaning of one's terms with a 
>>little care.
>>
>>You also ask how this is "useful";  my explanation above suggests 
>>the reverse -- monotonicity is in some respects an impediment.  But 
>>in an open-world environment, in which one can *never* be sure of 
>>having all of the facts to hand, no conclusions can be drawn 
>>without an assumption of monotonicity.  So if we are to make 
>>inferences in such circumstances, we must have a monotonic 
>>framework.
>
>Im sorry but I cannot grok this reasoning. 
>Granted, we cannot assume we have all the facts.

Right

>Granted, the truth of our conclusions are always dependant upon the 
>truth and *completness* of our premises.

Wrong. The truth of the conclusions does not depend on the 
completeness of the premises. That is exactly the point. If it did, 
it would be impossible to draw any conclusions, because you cannot 
know your premises are complete (unless of course one of them 
actually asserts this somehow.)

>  How can monotonicity change that predicament in the slightest?  I 
>keep hearing:  "We know that we cannot conclude such-and-such 
>because we might be missing some information, so we *must* change 
>all the vocabulary and inference patterns to  be monotonic,  and 
>then  we *can* draw such a conclusion.   Huh, what changed - what 
>suddenly made drawing unwarrented conclusions valid?   How can we 
>suddely make statements that are always true?

Its easy to make statements that are always true. One sure way is to 
make them linked to a particular time, and include the time in the 
assertion. Then if they are ever true, they are always true. Almost 
all temporal databases are full of stuff that is always true.

>>Having an monotonic, open-world framework does not, in my view, 
>>mean that one cannot also have a locally applied closed-world 
>>assumption for some applications, but this strays outside the 
>>globally specified behaviour for RDF.  And the conclusions drawn by 
>>such an application cannot be returned to the global semantic web 
>>(unless somehow qualified by some expression of the closed world 
>>assumption used -- which might be presented as a "context").
>
>But we have no way to present a "context"  with RDF  statements. 
>Everything in RDF seems always to be true regardless of its context.

Right. That is admittedly a simplification of the real world 
situation, but its not fatal, since we can always encode the context 
in the assertion and thereby render it non-contextual (or at any rate 
non-contextual enough to be useful.)

>Of course we all know that's absurb.  I really dont see how 
>monotonic logic helps us here - rather the other way around.  It 
>seems to lead people to believe that all of their RDF conclusions 
>will always be valid and that discovering new information cannot 
>change those conclusions.

The issue is rather more limited than this. When specifying the RDF 
standard, we need to give the thing a semantics which makes this 
universal assumption. If we do not, then ALL RDF inferences will be 
risky and unsound: any inference you make, anywhere, could be refuted 
by something you learn later. If we did this, almost the entire Web 
traffic would be taken up with "whoops, sorry, I was wrong about 
that"  messages.  All the spec does, in fact, is say that IF you use 
RDF content in a nonmonotonic way - for example if you assume that 
you have all the relevant information about something and assume that 
anything missing is false - then you are on your own. YOU have made 
this assumption, taken this risk. But of course you are free to do 
so: its just that the spec says, RDF no longer guarantees that your 
conclusions follow from your inputs, OK? The guarantee only applies 
to monotonic uses. Its like reading on the box: "Warning: if used 
indoors may cause risk of fire". It doesn't mean, don't use indoors. 
It just says: if you do use it indoors and it catches fire, don't 
come blaming us.  If you use RDF nonmonotonically and you get false 
conclusions from true premises, don't come blaming the RDF spec. We 
warned you on the box.

>If RDF is really specified like this, then only God will be able to 
>write it, because only God will be able to compute all the 
>implications of his\her statements forever and ever.

Sure, but I don't need to do that, only to guarantee their truth. And 
there are lots of facts than can be stated confidently as true for 
the forseeable future, eg lists of all the US presidents so far.

Pat


>
>What am I missing?
>
>Seth Russell
>http://robustai.net/papers/Monotonic_Reasoning_on_the_Semantic_Web.html


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Received on Thursday, 6 February 2003 18:12:41 GMT

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