W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: New issue - Meaning of URIs in RDF documents

From: pat hayes <phayes@ihmc.us>
Date: Tue, 22 Jul 2003 00:35:39 -0500
Message-Id: <p06001a10bb425d43924a@[10.0.100.23]>
To: Tim Berners-Lee <timbl@w3.org>
Cc: www-tag@w3.org
(sorry about delay in responding: lost a batch of halfwritten emails 
in a crash :-(  )

>>>1. " each URI
>>>identify one thing ("Resource": concept, etc)."
>>
>>Exactly what is meant by "identify" here is not exactly clear, but 
>>if this means something  close to what it usually means then it is 
>>simply untenable to claim that all names identify one thing.
>>
>
>>I am making the claim only for RDF statements in a global context, 
>>in for example an email sent between two people who don't know each 
>>other but both access to the web.
>>
>>
>>So am I: and I insist that this stipulation of identifying one 
>>thing isn't sensible or even desireable. Well, at least, unless 
>>that word "identify" means something different from "refer to" or 
>>"name" or "denote" .
>
>I think "denote" probably matches.  I will try to use "denote".
>
>>   What might indeed be true is that in many circumstances, a URI 
>>somehow provides access to information which is sufficient to 
>>enable someone or something to uniquely identify a particular thing 
>>(that the representation accessed via that URI is in some sense 
>>about), but even there the thing identified might vary between 
>>contexts (such as when we use someones email address to refer to 
>>the person) without harm.
>
>This depends on what you mean by "contexts".  If you mean that I can 
>send one person an email saying (in RDF)
>   <http://example.com/foo.rdf#bar>  pantone:color  "blue426" .
>and it can mean one thing and I send it to another person it can 
>mean something else,
>then we do not have system of communication which has any properties at all.

Well, no. What we have to do in order to communicate is to make sure 
(as far as we possibly can) that the RANGE of possible 
interpretations of what I say also captures the range of possible 
interpretations of what you believe. That is what it means to say 
that 'you understand what I mean' or 'we mean the same thing'. Thats 
not the same as saying that there is only one possible interpretation 
of what we say to one another, which is almost never true. Even in 
this Pantone example there is room for some variation in meaning (see 
my response to Patrick's reply to your message).

>
>>This kind of ambiguity resolved by context is at the very basis of 
>>human communication: it works in human life,
>
>Yes, with natural language and peotry

Never mind poetry, it works for all communication. There are all 
kinds of examples noted by the sociolinguists, such as the waiters 
habit of referring to the customers by the dishes they order ("The 
hamburger on 7 has a nice haircut") but in fact its ubiquitous. YOu 
notice it sharply when you try to do exact ontology work, because the 
range of meanings starts to get narrower, and the number of possible 
misinterpretations increases, when you get more exact, eg 'bank' in 
English is often said to have 3 meanings but if you start getting 
precise its more like 10 (aircraft turn, aircraft attitude, shape of 
a curved surface in oriented 3space, edge of a waterway, dynamic 
position of a turning vehicle, financial institution considered as a 
commercial agent, similarly considered as a taxable entity, a 
building, a location in a city, a group or set of buildings with a 
common owner, ...) and if you need to distinguish these, then 
suddenly what seemed like a single meaning dissolves into an 
ambiguity which needs to be resolved somehow. Its *impossible* to 
resolve these all ahead of time once and for all, because how many of 
them there is to resolve itself depends on the context. There isn't a 
predefined set of meanings lying around which can be uniquely 
identified by URIs; they have to be allowed to appear as new things 
are said. Even apparently 'atomic' senses need to be allowed to 
become negotiable, eg see the possible ways to understand color terms.

BTW, one reaction to this observation is to say that its an artifact 
of the formalisms we use. Maybe it is, but these are the only ones we 
have got: and in any case, the linguistic semanticians have noticed 
the same phenomenon when trying to give a semantics directly to 
English (or Dutch, or whatever); they call it the problem of 
underdetermination of meaning. It seems to be pretty basic.

>
>
>>  it works on the Web,
>
>Yes, when the genre is natural language and peotry, not mathematics,

No, it happens in mathematics as well, there's even a term for it, 
its affectionately called 'abuse of notation ', as when we identify a 
singleton set with its only element, or a trivial structure with its 
generating set. Its often done, sometimes without detailed 
explanation, you see things like 'we use familiar abuses of 
notation'. .

>
>>  it will work on the semantic Web.
>
>No.  We are defining the semantic web NOT to work like natural language,
>but to work like mathematics.
>
>And it does not work in math.

See above. And I don't think that we are making the SW like 
mathematics, in any case, NOne of the SW languages are remotely like 
mathematics: they are formal logics, not languages for doing 
mathematics.

>
>Suppose I give you two facts, that x=1 and that x=0. Not a problem, if
>one can assume the x denotes something different in the two cases.
>But very hard to build any logic at all.

No, look. If I say that x=0 and you say that x=1 then clearly we 
disagree about x. BUt that isnt the same as saying that 'x' must have 
a single, fixed denotation in all interpretations.  We might disagree 
about some facts without even knowing what the denotations are. If I 
say that the guy over there has red hair and you say no, his hair is 
brown, then we are still disagreeing, even though neither of us has 
the slightest idea who he actually is.

>
>>Why do you want to try to legislate it out of existence?
>
>Any system of mathematics has to be able to use symbols to denote things
>in the universe of discourse.

True, but beside the point.

>You as a philosopher can perhaps handle
>a mathematics in which symbols denote whatever anyone likes at any point,
>but I as an engineer find it less useful.

Im not saying that denotations are arbitrary or at whim; but to deny 
that is not to say that they have single, fixed interpretations.

>
>>  You will not be able to, any more than you will be able to stop 
>>people falling in love.
>
>Ah, but people have stopped falling in love. Look up ... one by one 
>the stars are going out. ;-)

You had me worried there for a second.

>But seriously...
>
>>All that your 'ideal design' will accomplish is to make the 
>>architectural pronouncements of the W3C more and more out of line 
>>with the way that the Web is actually being used by real people.
>
>People are not using the semantic web now.
>There is not very much global math on the web.
>People use document identifiers as though they (in some sense) will from
>week to week denote (in some sense) the same thing, so people are very
>used to having a global space of identifiers.

People are not very used to having to make very fine discriminations 
of meaning, though. In fact it is very hard indeed to get them to do 
this, and its almost impossible without some training.

>>Take your example of person A emailing person B, who A does not 
>>know. What is actually going on here, described precisely, is 
>>surely that A knows that 'B@Bsplace.org' is a character string 
>>which when used in a certain way will  (by some occult technical 
>>means about which A need know very little) act as an address, so 
>>that email sent to that address will arrive in the inbox of, and 
>>likely be read by, someone called 'B'.  I phrase it thus because it 
>>might be potentially misleading to just say 'read by B' since that 
>>could be understood as saying that A knows the referent of that 
>>name 'B'; but we are assuming that A doesn't. So what A knows is in 
>>fact an existential: that a person called 'B' *exists* who will get 
>>the email.  Since A knows that the email exists and is unique - A 
>>has direct acquaintance with the email, having written it - this is 
>>enough for A to know that there is a single person out there who 
>>will get the email.  But it is still misleading to say that the 
>>email address "identifies" B: if that really were true, then A 
>>could find out who B was just by looking at the email address.
>
>This is rather tangential.  My example was of people emailing each 
>other, and the content of the email having the semantics to A as to 
>B.  You discuss who is denoted by "B@Bsplace.org", an email address. 
>The email address, (or we could tak of the related URI, 
>"mailto:B@Bsplace.org ") in the semantic web, denotes, formally, 
>something often referred to as an "RFC822 mailbox", and which is a 
>conceptual thing to which  mails may be sent to or from, among other 
>uses.  There is a relationship, one of whose URIs is
>http://www.w3.org/2000/10/swap/pim/contact#mailbox
>which relates a social entity (for example, a person) to one of 
>these mailbox things (as a mailto: resource). People often use the 
>approximation that contact:mailbox is inverse functional,
>allowing them to determine that two people are the same person 
>because they have
>contact:mailbox, but that does not mean that in the formal system we 
>are building to represent all this, that the "mailto:B@Bsplace.org" 
>denotes the person.

First, there isnt any particular technical reason why we need to make 
rulings about things like this. More to the point, however, the 
system we are building has to be useable by real people, and real 
people *will* do things like this: one of them will use URis in one 
way, others in another way, maybe systematically related. There 
really is no way you or anyone else can stop them, so its no use 
trying, you will just be king Canute with the tide. Rather, we should 
be trying to provide ways for them to recognize, work around and 
negotiate the kinds of differences that they are likely to come up 
against. See 
http://cs-www.cs.yale.edu/homes/dvm/daml/ontology-translation.html 
for example.

Consider a case where one ontology for reasons of its own uses 
mailto: URIs to denote people. As long as it is internally 
consistent, no harm is done. NOw suppose it communicates some of 
itself to B, which uses mailto: URIs to denote mailboxes. Pretty 
quickly, B is going to notice some oddities, such as concluding that 
mailboxes are in an impossible intersection of web resources and 
animate objects with parents, say.  What to do? An immediate fix is 
to completely centextualize the other ontology, not trusting any of 
its terms to be the same, then try to connect them one by one until 
something breaks agaiin, then back up and look for a resolution 
(there are smarter ways of handling it) in the form of a likely 
mapping from A:mailto's to B:animate agents. This might require 
accessing another ontology of 'likely mappings' or it might use a 
more sophisticated kind of structure-mapping which can handle common 
metaphors, such as Gentner & Forbus' SME method. Either way, this 
kind of quick repair to a communication is going to be required and 
is within the state of the art.

>>And I am describing, if you like, a perfect platonic design, to 
>>which we can aspire, though social and engineering factors limit 
>>our ability to implement it perfectly.
>>
>>Allowing - no, admitting the existence of - referential ambiguity 
>>is not an imperfection: it is a basic property of communications of 
>>belief using language, one that is recognized and even described 
>>quite well (to a first approximation) by the model theory that you 
>>dismiss.
>
>I do not dismiss model theory, I just pointed out earlier that your 
>questioning of the use of "identifies" rather than "denotes" was 
>asking me to use MT terms rather than other english terms.
>
>Now the model theory I have seen only describes the semantics of the 
>OWL terms, in explaining how the  statements that Fido is a dog and 
>a dog is a subclass of animal constraint the possible 
>interpretations.   And this is done so as to work on any valid 
>interpretation.  I have not seen ( but I may have missed) the bit 
>where when the english in a schema describes what the individual 
>ex:fido is, that interpretations are further constrained to those in 
>which "ex:fido" actually denotes the actual dog we all know and love 
>as Fido.

There is no such bit. There can't be; you won't be able to provide 
it, either. (And you might have some problems doing in math in any 
case. :-) Certainly just *saying* that URIs have unique 
interpretations isn't going to do it.

BTW, this is what I meant at the Plenary village green by saying that 
there aren't any names on the Web. That's because to attach a name to 
Fido, you need some kind of naming protocols involving the actual 
dog: a way to baptize things, in a sense: and the Web has no baptism 
protocols.

>
>>>Like with all technical specs, the fact of imperfect adherence in 
>>>some cases does not detract from the importance of having made the 
>>>perfect idealistic design which has provable properties. One deals 
>>>with deviations from the perfect in a form of perturbation theory.
>>
>>
>>We seem to be at cross purposes. Im not saying that the 'unique 
>>identification' condition is an unattainable ideal: Im saying that 
>>it doesn't make sense, that it isn't true, and that it could not 
>>possibly be true. Im saying that it is *crazy*.
>
>Well, you have used "silly" and "crazy", but in the context of your 
>statement they clearly denote the characteristics of being well 
>thought out and essential to the architecture of the semantic web, 
>respectively.

I disagree. The unique identification condition isn't well thought 
out and its not only not essential to the architecture, but the Web 
wouldn't work (or more properly, would be totally irrelevant) if it 
were true.

>
>>
>>  Existing W3C standards already provide counterexamples: what 
>>single thing is identified by the URI reference 
>>http://www.w3.org/2000/01/rdf-schema#Class? This is supposed to 
>>*denote* the class of all RDFS classes; but that is not a single 
>>well-defined notion, by the very nature of formal semantics: it 
>>varies from interpretation to interpretation.
>
>An interpretation is  a mapping from names to things.
>What I am saying is that, if there are two interpretations, and the 
>things denoted by that URI in those two implementations are 
>demonstrably different, then it is reasonable to go back and ask the 
>owner of the URI which one is denoted.

No, its not reasonable to do that. In fact its probably impossible: 
how would you refer to those alternative interpretations? Suppose 
ex:thisURI has two possible interpretations. What do you say, to ask 
which is correct? About the best you could do is to ask about an 
identity: "is it true that ex:thisURI= ex;thatURI ?" Suppose the 
answer is Yes.  Now you know that in any interpretation, ex:thisURI 
and ex:thatURI denote the same thing. You still don't know which is 
the right interpretation, though.

>  The authority may decline to reply of course, but if it thinks and 
>thinks and comes back with an answer, then that answer is added to 
>the common information which we share, and one of the 
>interpretations has to be dropped.
>
>>And there is the problem that MT systems consider all possible 
>>interpretations of the data, in any possible worlds.
>>
>>That is not a PROBLEM; it is how semantics works. When you 
>>communicate something to me, you send me some language (or more 
>>generally some representations). I have to try to interpret this 
>>language and make of it what I can.  But you cannot POSSIBLY send 
>>me a single interpretation: interpretations are not the kind of 
>>thing that can get communicated. Only language gets communicated.
>
>Indeed, to communicate what something denotes one would need magic.
>Like telling a robot - you want to know what "hot" is? this is hot. 
>And stimulating its temperature sensor.

Exactly; you need to get the terms 'grounded' somehow, as folk often 
say. Not magic, but not semantics either.

>Communication doesn't allow any terms which everyone understands. 
>Everything communicated is only a message, and the receiver can only 
>sense the message and never know what it means. Nothing has
>fundamental meaning, a message will just have certain effects on 
>certain agent, and agents will change their internal stored state as 
>a result of them.
>
>>  So yes, OF COURSE there are many possible interpretations of what 
>>you say, even when I have used all my resources of interpretation. 
>>This isn't a problem of the theory, it is a FACT ABOUT 
>>COMMUNICATION which the theory recognizes and tries - in admittedly 
>>a crude way, but we have to start somewhere - to deal with and come 
>>to terms with.
>
>This is theory of communication does indeed address how 
>communication.  However, different theories are used at different 
>scales, and different stages in the analysis.
>
>[When we analyze how an electron behaves, we use quantum mechanics. 
>We discover that the position and momentum of the electron cannot be 
>known at the same time.   This is just a fact about matter, which 
>the theory recognizes and tried - in admittedly crude way, but we 
>have to start somewhere - to deal with and come to terms with.  .... 
>We realize that application of that theory in great detail will 
>allow us to make a wave equation for an apple, and we figure out 
>that (though it is too complicated a job to do in practice) in any 
>reasonable approximation, when considering 10^^23 particles, the 
>result is that an apple has, to all intents and purposes,  a given 
>position and momentum at any time.  It isn't that quantum mechanics 
>doesn't hold for apples. It just isn't worth doing it when using 
>real apples.  As we take a bite, the theorist jumps up and
>down warning that it could jump sideways at any moment.

If the theorist is any good, she just bites, having figured out the 
odds of it jumping are so low that it would take more than the age of 
the universe to reach anything measureable.

>The engineer takes the bite.]
>
>So let it be for the semantic web.

You have given me an irrelevant parable, and you say 'so let it be'? 
So let it be *what*?

>Many agents have communicated at great length over what the URI 
>daml:TransitiveProperty denotes.  During this process, the people 
>involved considered many interpretations.  Not a ridiculous number, 
>as few in the working group considered interpretations in which 
>daml:TransitiveProperty denoted the dog we all love and know as 
>Fido.  But the process which you describe in capitals above took 
>place.

I was referring to a different process, but OK.

>Drafts were written.  Textbooks written ages a go and read by many 
>were quoted.  By the end of the process, after axiomatic semantics 
>had been written up and reviewed, and a model theory had been 
>written (in english), people went away and wrote programs which 
>treated daml:TransitiveProperty in a particular way.  People found 
>that when one program generated a statement about something being a 
>transitive property, the other program did good things.   Now, no on 
>can say that the people wring those two programs had the same 
>interpretation of the spec, and really in theory shared a common 
>thing as that denoted by the URI.  But for that and several other 
>URIs, the proof was in the eating. The programs worked. And will 
>work, for lots of other people in the future.
>
>It is as though that bit of magic has happened.  When you and I 
>write an ontology for marsupials, we don't worry about differences 
>in what we mean by "subclass".

Agreed, but the fact that we don't worry about them doesn't mean that 
they aren't there: it just means they don't matter for the purposes 
in hand.

>  We only worry about what we mean by "duck-billed platypus".  When 
>we have finished our ontology of marsupials, and a thousand experts 
>have poured over it and written commentaries on it, then millions of 
>school kids will happily refer to the class of marsupials using our 
>URI.   The arguments will have been done.  From the standpoint of 
>the school kid, the class is  a well-defined concept, where 
>linguistic processes have long since tended to an asymptote, and any 
>misunderstandings can be dealt with

Well, interesting example. In fact, not quite true: rdfs:subClassOf 
has slightly different semantic conditions in RDFS than in OWL - but 
not so you'd likely notice when thinking of platypuses, no doubt.

But suppose all this is true: none of this says that we have agreed 
on a single interpretation of rdfs:subClassOf, or of 
daml:TransitiveProperty. What we have all agreed on is the SEMANTIC 
CONDITIONS imposed on any interpretation of these things; the 
conditions expressed in the model theory and which sanction the 
useful inference rules that are built into the software. But if we 
really had agreed on a single interpretation of these terms, we 
wouldn't need inference rules or software: we would already know, of 
every two things in the universe, which were subclasses of the other 
and which were not. An interpretation is an entire WORLD, completely 
specified down to the last jot and tittle. Knowledge of that is 
indeed TOTAL knowledge: if there is only one possible interpretation, 
then no inference or even thought ever would need to be done: we 
would all be omniscient.

Im sure that you didn't mean to imply this, as it is completely 
crazy; but  that is what you have been *saying*, honestly.

>
>
>>  If you take the  case of an identifier for  pat hayes 
>><phayes@ihmc.us>, for example, the non-logician would consider that 
>>it identified one person and get on with their lives
>>
>>
>>The logician can say that also: it is the assertion that a single 
>>person exists who has that name. But (1) that is not the same as 
>>saying that the name - all by itself - "identifies" a single person 
>>(or, well, maybe it is: but if so, then other things said about 
>>URIs and resources are wrong) and (2) in fact, they don't assume 
>>that and get on with their lives. Sometimes they assume it 
>>indicates a person, sometimes a mailbox, sometimes a computer: it 
>>depends on the context.
>
>With strings, yes, not with URIs.

No, with URIs as well.

>
>There two reasons you are being confused.
>
>1) Sloppiness.  Human beings refer to things through the values of 
>properies all the time  ("ask fancy pants what he asked 411 for") , 
>and figure  out what people mean, in english but not in math.

In math too, see above.  And its not sloppiness, its efficiency. 
There are good reasons why people do this: it would be impractical 
not to. Nobody could keep track of the distinctions that would need 
to be made in order to make the simplest assertion if this 
'sloppiness' weren't available.

>
>2) Confusion with times when the design is to specifically and 
>unambigously use a name of one thing to indirectly point to 
>something else.   You give an email address of a person who is going 
>to attend a conference.   The email mail box isn't going to attend 
>the conference, and everyone knows that there is unambiguous 
>traversal of a "contact:mailbox" arc involved.   People are confused 
>because a namespace (whatever that is)  is indicated by giving the 
>URI of a (maybe notional) namespace document which corresponds to 
>that namespace. As the namespace is kinda abstract, and only the 
>namespace document can be measured, this doesn't really matter.
>
>
>>Which is fine, let me quickly add, provided some 
>>bull-in-the-china-shop authority doesnt keep insisting that all 
>>URIs must by fiat always identify a single resource. Then we get 
>>interminable arguments and discussions about what 'the' resource is 
>>in this very case, and the people who are insisting on this 
>>doctrine so firmly tend to be the ones who get exasperated earliest 
>>and tell us that it doesnt really matter what the "resource" 
>>actually *is*; apparently missing the irony of the fact that the 
>>only reason we are having this argument is because of this insane 
>>ruling that they are so insistent upon not budging from. Grrrr.
>
>Grr indeed!  To what extent must we settle what the resource is?
>That is probably the question which divides our positions.
>I would say that when we have more than one candidate and these 
>candidates are incompatible, ambiguity would lead to inconsistency, 
>then we must settle it.

I fail to see how ambiguity can lead to inconsistency. It might (and 
sometimes does) lead to errors, I admit.

>
>Example1.
>
>A dog bounds into the room. Tim says, "Here, Fido!" to the dog, and 
>says "Pat, meet my dog, Fido" to Pat. Tim plays with th edog. Tim 
>asks Pat, "Pat, would please take Fido for a walk?"
>Pat takes the dog for a walk.  The name seems to have been 
>unambiguously associated with te same dog in both there minds.
>
>Example 2.
>
>Two dogs bound into the room. Tim says, "Here, Fido!" to the first 
>dog, and says "Pat, meet my dog, Fido" to Pat. Tim plays with the 
>second dog. Tim asks Pat, "Pat, would please take Fido for a walk?"
>Pat has to ask which dog is Fido.  The name was not unambiguously 
>associated with the same dog in both there minds.  Pat hat to fix 
>that before he could continue the conversation.
>
>Example 3.
>
>A dog and a cat bound into the room. Tim says, "Here, Fido!" to the 
>first dog, and says "Pat, meet my dog, Fido" to Pat. Tim plays with 
>the first dog. Tim asks Pat, "Pat, would please take Fido for a 
>walk?"
>Pat takes the cat for a walk.  "For me, in this context, 'Fido' 
>denotes the cat.", he says as he leaves.
>
>Which scenario is insane?  The first two?

What was the cat's name, in example 3?

Im not sure what point you are making by this, so let me tell you a 
true story to illustrate my point. Once I was in a room with two 
people making a list of the things in the room. The conversation went 
something like this, with A looking around and B typing.
A: Desk, chair, curtains, ..... , picture....
B: The picture's not in the room.
A: !!!? Of course its in the room. Im looking at it.
B. Its part of the room, not in the room.
A> Wha??
there then followed about an hour's intense, often heated discussion 
about what was or was not in the room, ranging over the paint on the 
wall, the fitted carpet, the door, the door when open, etc. etc. 
Eventuially, slowly, we figured out that they had different notions 
of 'room': A's idea was the shell left by the building contractor, B 
was something like a decorated, inhabitable space. But the point is 
that these two adult native speakers, who knew one another well, had 
never discovered that they had this conceptual mismatch. Each of them 
obviously thought the other one was totally weird. If they had been 
obliged to be formal-ontology clear and unambiguous in what they said 
to one another, they would have discovered it years before. But until 
they started writiing axioms, it hadnt ever mattered before.

There is lots of evidence, I will go so far as to say overwhelming 
evidence, that virtually ALL our use of language is like this. We 
DONT all have the same concepts in our heads, and we don't easily or 
naturally have our concepts align perfectly with those used by other 
people. When groups and individuals start writing down their ideas at 
DAML- or OWL=level precision, they will not agree on exact meanings, 
and there is no point trying to make them do so. They will have to do 
some content negotiation to work things out. The fact that we 
communicate successfully in natural language is beside the point, 
even if it is true, Our ontology languages require better than 
natural-language-agreement conceptual precision in order to work.

>
>
>[...]
>Pat:
>>>That is, as we add information about it, that information should 
>>>not be inconsistent.
>>
>>
>>Right. MT helps you there by providing a crisp notion of 
>>consistency. It also gives you an important insight: if you know 
>>enough to uniquely identify the referent of a name, then *any* 
>>further information is either redundant or inconsistent. Basically, 
>>this follows from the observation that the only proper subset of a 
>>singleton set is the empty set.
>>
>>You can think of it denoting different things in different systems, 
>>but how are those things "different" apart from the fact they are 
>>in different systems?
>>
>>Well, how are they the same? That is, what gives us a licence to 
>>claim that rdfs:Class and owl:Class, for example, are the same 
>>class? (In fact, there is a good reason in this case to say they 
>>are not the same.)
>
>The are not the same because owl:Class is a member of rdfs:Class but 
>not of owl:Class, n'est-ce pas?

You could describe that as a symptom, yes.

>For that reason I would say that it would be broken to use the same 
>URI for the two classes.

I tend to agree, but not because there is more than one 
interpretation, but because the semantic conditions are different.

>
>>Maybe you have a more direct acquaintance with abstractions like 
>>the class of all classes than I do, but I sure wouldn't know how to 
>>decide things like this in general, and I *know* that no computable 
>>decision procedure could decide it for me.
>
>But no one asked for a computable decision procedure.
>The corners of math can throw up lots of tricky things which make 
>people nervous, but fortunately the bulk of semantic web traffic 
>will be in terms of things like date, totalamountinusdollars, 
>financial instution identifier, etc.

Dates, hmm? You picked a tricky case there to get right.

You seem to be operating under a kind of broad-lapels Yorkshire kind 
of confidence in the practical: we just need to get down to brass 
tacks and it will all be clear, none of this silly academic 
shillyshallying about over bloody theorems and the like. But it 
won't. Honestly, now, his is where we AI folk, for all our faults, 
really do have some practical experience. You DONT get operating 
ontology software just by writing down the obvious practical stuff 
that everyone knows, like dates and total amounts. First, people 
don't agree about what things like dates really are (is a month a 
kind of duration? Think carefully before you answer). Second, even 
when they more or less do, after a *great* deal of discussion, 
deciding how best to formalize it is still an art, and there are no 
ground rules or even broad consensus for many - most - topics. Things 
as simple as how to describe a river flowing to the sea, or a 
tropical storm, are still research issues. And third, there are no 
agreed-on upper level conceptual frameworks to fit all this stuff 
into; existing deployed ontology standards do not agree. . In many 
ways, the 'simple' things that everyone knows are the hardest to 
formalize, sine we don't normally have to even articulate them.

>>>  We say every owl:class is an rdfs:Class. That allows us to deduce 
>>>things about some classes. Suppose we make other assertions about 
>>>rdfs:classes, is it allowable for us to be able to make a 
>>>contradiction? I would say not.  Currently, different logical 
>>>systems can deduce different things, but the important point is 
>>>that they are talking about the same thing when they use the same 
>>>URI.
>>
>>
>>You need to be careful what you mean by 'same thing'. Sure, if 
>>reasoner A uses 'rdfs:Class' and reasoner B uses the same URI, then 
>>they ought to both be using the name in the same way, so that they 
>>can communicate.     . . . . . . [12]
>
>Yes indeed.  I guess that is what I wanted you to say all along.

I never said anything different. The model theory even makes the 
required sense of 'same way' precise.  On the other hand, as Ive 
tried to make clear, I think that even this is an idealization which 
we will not be able to achieve in practice.

>For all B.
>
>>  Nobody is disagreeing with that.  But that is not the same as 
>>saying that there must be a single thing that this URI is naming. 
>>Analogy: if we hold hands then we are walking the same way.  But 
>>that does not mean there is only one way we can possibly walk. I 
>>think you mean the former, but you are saying the latter.
>
>So you accept that everyone must treat the identifier in the same 
>way, that perhaps we could say that for two people it must identify 
>the same thing, but not that there is one thing which is identifies?

For two people it must identify the same thing in every 
interpretation of their shared communication.  There's a nice term 
for that: the 'common ground'; the stuff they agree on. That is still 
not saying that there has to be a single thing that it identifies for 
both of them.

>Can we not show that the two conditions are the same?

No, because they are obviously not the same.

>Suppose there was not a single thing denoted by the URI.  Then there 
>must be two distinct things denoted by the URI.  Those things to be 
>distinct  must be such that is an A uses one and B another as the 
>referent of the URI in a message between them

Not so. That could happen only if the message could somehow 
distinguish the two interpretations. BUt there is only one URI, and 
even if the message contains it, then the message itself could be 
ambiguous. There need be no way for A and B to discover the 
ambiguity; they might agree on all their beliefs, and still there 
could be more than one possible interpretation. Suppose you tell me a 
million equations and I check them all out and agree they are all 
true and tell you a billion more in return, and you check all those. 
We agree on everything: still, we have only mentioned a few billion 
numbers, and there are *infinitely* many integers in any 
interpretation of arithmetic. We havn't got time to check infinitely 
many equations. so our extended conversation still allows for many 
different interpretations. We could exchange facts about arithmetic 
until the rocks melt wi'the sun, and still we wouldn't have succeeded 
in nailing down our universe of discourse to a *single* 
interpretation.

>, A and B behave inappropriately and so the system is broken.

The system works fine, and doesn't require a single interpretation. 
Believe me, I really do know this stuff.

>
>We have honed this distinction down to a faction of a hairs width now.

Well, this is rather important: the difference between agreeing on 
the facts, and agreeing on a single interpretation, is kind of basic. 
Thats why Ive been naggling about this so determinedly (and not just 
to you, BTW.)

>>[...]
>
>>Perhaps 'identify' doesn't mean 'denote' or 'refer to'. What does 
>>it mean, then? Note that if we were to say that 'identify' means 
>>MORE than simply 'denote' or 'refer to' - if, say, it also has a 
>>connotation that the URI can be somehow used to retrieve some 
>>information about the referent - then the claim would become even 
>>more false.
>>
>>
>>When one retrieves a document, one gets information which its 
>>publisher says, and one can believe or not.  But using a term does 
>>(modulo social things such as fraud and engineering things such as 
>>broken cables) commit you to the term owner's definition of it, and 
>>the document they publish at its URI is taken by design to be 
>>information deemed shared by those using the term.  That's the 
>>contract.
>>
>>
>>Im happy with that contract, though with a slight hair-tingle at 
>>the use of the word 'definition'. But nothing in there says that 
>>URIs must uniquely identify resources: in fact, you didn't even use 
>>the words "resource" or "identify" , which I am very happy to see 
>>were also missing from Tim Bray's down-to-the-wire summary of the 
>>essential core of things.
>>
>
>
>
>>[....]
>
>>>>First, OWL is more than an RDF vocabulary: it is an RDF 
>>>>vocabulary with a particular semantics applied to it.
>>>
>>>Like every RDF vocabulary.  What is interesting about OWL is that 
>>>for some of the vocabulary the properties of the Properties can be 
>>>defined in math.  But basically OWL isn't any different from the 
>>>calendar event vocabulary.  The only reason that an RDF calendar 
>>>event has meaning is the semantics of that vocabulary.
>>
>>As you know, I disagree profoundly with you on this issue. The 
>>semantics of an calendar event described in RDF is given by the RDF 
>>vocabulary. It is axiomatized in RDF. You can write as much as you 
>>like about it and what you think it ought to mean: all that is 
>>merely commentary and does not change the meaning *of the RDF* one 
>>iota.  That follows from the RDF specs themselves.
>
>I don't follow. Imagining schemas and specs  where appropriate, what does
>
>[]  rdf:type  cal:Event;
>     cal:dtstart "2003-07-31T12:00:00Z";
>     cal:end "2003-07-31T13:00:00Z";
>    cal:participant   [ contact:mailbox <phayes@ihmc.us> ].
>
>and why?

Sorry? What does it what? (Mean? entail?)

>
>
>>>(For you, this may seem perturbing or to say that the logic 
>>>itself, the thing you tend to define first, is actually only 
>>>defined in the data language. But it works.
>>
>>No, it doesn't, which is why I insist on my point.
>
>Does too.
>__________________________________________
>
>At this point I am horrified to find myself only a fraction of the 
>way through the email conversation, so I will hit send and keep the 
>rest for another day

I know that feeling.

Pat


>
>Tim.


-- 
---------------------------------------------------------------------
IHMC	(850)434 8903 or (650)494 3973   home
40 South Alcaniz St.	(850)202 4416   office
Pensacola			(850)202 4440   fax
FL 32501			(850)291 0667    cell
phayes@ihmc.us       http://www.ihmc.us/users/phayes
Received on Tuesday, 22 July 2003 01:35:49 UTC

This archive was generated by hypermail 2.3.1 : Wednesday, 7 January 2015 15:32:38 UTC