Short answer:
Yes there is meaning to RDF statements which is not expressed in RDF.
Long answer:
On Wednesday, May 29, 2002, at 03:43 PM, patrick hayes wrote:
----- Original Message -----
From: "Brian McBride" <
To: <
Cc: "RDF Core" <
Sent: Tuesday, March 26, 2002 1:38 PM
Subject: help wanted: RDF issue rdfms-assertion
Tim,
The RDFCore WG seeks your help with an RDF issue, rdfms-assertion:
http://www.w3.org/2000/03/rdf-tracking/#rdfms-assertion
[[
Summary: RDF is not just a data model. The RDF specs should define a
semantics so that an RDF statement on the web is interpreted as an
assertion of that statement such that its author would be responsible
in
law as if it had been published in, say, a newspaper.
]]
The WG believes that this issue originates with you.
I would like to clearly establish what it is that you would like from
us.
A number of concerns have been raised about this issue:
o RDF is just one of several specifications that are 'in play'
when an
RDF statement is retrieved from the web. What is the minimum the RDF
specs
must say to achieve the effect that you want.
I think that it should say that the predicate determines the meaning
of any
statement.
? What does that mean?
[See below**]
(I can only interpret it as something that is obviously
false, so I must be misunderstanding it.
Thank you.
Suppose a document in RDF parses to the following statements:
_:b dc:title "Moby Dick".
_:b dc:creator _:c .
_:c contact:email <.
People actually write documents which parse to statements like that.
Libraries work only because people accept that such things have
meaning.
These statements are taken to mean that you are creator of something
called "Moby Dick". Leaving aside niggling over details, time, etc,
you have to go along with that.
If you are, they are true; if you are not they are false.
If, for example, I accept the assertions of a library card to this
effect, then to be simple I now know you wrote such a thing. To be
complicated about it, the possible worlds which I can consider this
world as being among have been constrained to those in which you did
write such a thing. (I am sure one could be much more complicated -
but that is not the object of the exercise!))
Now Peter's RDF engine will of course not know this, as you can't
describe creatorship in RDF from scratch, and he's not prepared as
calling peter:RDF anything which involves inference using anything
other than the RDF spec. Which is limiting, and not what RDF is for.
Peter's interest in the set of inferences you can make from a
statement is of course useful, but saying that there is only one set
is not. In fact, they hard fact of life on the web is that you learn
stuff, and some people know more than others, and some machines have
greater capabilities than others.
Let me show you what I mean in another way-- in the negative -- what I
want to exclude.
Above, dc:creator is the predicate. It is defined, (say) by the
dublin core
that the meaning of a statement x dc:creator y is that x wrote,
painted or
otherwise created a work y.
Now suppose I define the resource w3c:tim in a namespace I control,
as
follows:
1. w3c:tim is a person, with email address <
2. w3c:tim is defined such that any statement of the form w3c:tim
dc:creator :y means that Pat Hayes should immediately send me a crate
of claret.
This would mean that anyone who uses the dublin core vocabulary to
describe me would end up, by my definition of the term which I have
introduced, stating that you should send me some wine.
What's the problem? Informally, it violates the actual practical way
RDF works, and in fact english too. The verb defines the meaning of
the sentence. In fact, a huge amount of what we do we use rules
which select on the predicate, and the idea that there be some subject
or object out there which feels it can override the meaning of the
predicate would cause everything to fall down.
Now most of the meanings of Properties ar currently defined in
english. The exceptions may become a formal derivation of numbers, but
I don't think even then that will be used in practice.
In a level where you are not looking at what information comes from
where (you have no quotation) and the only rules are the ones from a
fixed set of specs, this of course does not apply as you are not
actually saying anything about the real world or using any real world
classes or properties. And then when you slip into using RDF to talk
about the real world, you are so used to looking to the predicate of a
triple for meaning that you don't even realize you are doing it. But
we do have to formalize it so that it is possible on the sweb for
someone to introduce a term and define it. Even though the logic
doesn't have separate language for "is defined to be".
It should specify in the specific case of predicate
rdf:type that the
definition of rdf:type is that the object determines the meaning of the
statement.
It cannot say that, because that is either false or meaningless.
Rdf:type simply asserts that something is in a class; it does not
define "meaning" (either of the term denoting the object or the
expression denoting the class)
The way I am using "meaning" here, neither of these have it. Only
formulae have meaning (or, loosely, documents which parse to formulae).
and it doesn't really "determine" anything, since at best it
can only be said to *constrain* possible meanings, rather than
uniquely determine a meaning.
It should then hand off to the URI spec to say that
"determines" above means
that those publishing issuing or owning terms are the ones who
definitively
define (through specs etc) what they mean.
But that is impossible in RDF, since the full 'meaning' of a term
depends on the total graph of which it is a part, and this may have
been assembled from a variety of sources. So these sources all
contribute to the 'meaning' of the term; it cannot be said to derive
solely from any one of them.
Issues of
ownership, and dereference are covered not in the RDF spec but
directly or indirectly in the URI spec.
o Whilst the RDF specs might say what a statement means,
that meaning
might be modified by its context. For example, what about an RDF
graph
entitled "Myths about Namespaces". Would the publisher of that graph
be
asserting the statements therein?
*** The role of the RDF spec is to state what an RDF document means.
***
What the RDF spec says is that the meaning of an RDF graph is
expressed by the RDF model theory, and what that means is, roughly,
that the meaning of an RDF graph is expressed as a constraint on the
world being described, that it must make the graph true. Each document
defines a graph.
Saying that the meaning of the graph is expressed by the MT does not
help me. Not when the document parsed to form the graph is a check
(cheque) and the bank says it doesn't mean anything.
In the long term, bits of MT will no doubt be added which relate a
check to other terms in useful ways, but we have to start (and end)
with english definitions of key things.
If you have something more in mind by 'state what an RDF
document means', then please tell us what it is. If by 'meaning' you
mean something like a single unique meaning that can be determined
computationally, then it is impossible for any spec to provide it.
That is not a reasonable expectation for any formal specification.
Yes, you cannot determine or express the meaning of dc:author
computationally unless you just link it to other terms which are just
as impossible.
The role of SMTP spec to say that a message
delivered under certain
circumstances is
a message from one party to another.
In other words, other protocols deal with the question of who is
asserting
what.
Typically these things are complex and recursive, but the common point
of
reference is the meaning of a document.
I really think it would be more useful if you didn't take this line,
as it doesn't lead anywhere. There is no such thing as THE meaning of
a document, particularly of a document in a formal language.
(Speak for your own documents! ;-)
It is curious that above you ask me,
"0000,0000,DEDE? What does that mean?" of a
document I wrote. It looks as though you would like there to be a
meaning.
RDF is not just a formal language. It has ground terms which are
grounded in english (etc) prose which relate to the real world. That
is how it works. That doesn't invalidate the logical operations.
There are several precisely defined variations on 'meaning'
that you could use: for example, we can say what it means to draw a
valid conclusion from a document, or what it means for two documents
to be consistent with each other, or what it would mean for a document
to be true; but we cannot say what a document 'really means'.
I want to say what a document means.
You say I can say what it means for a document to be true.
I think that would be the same thing.
I think to say what it would mean for the fact that a document is true
to be true would mean the same thing.
So, maybe your philosopher's fear of the word "meaning" would be
averted if you consider "what a document means" to be a shorthand for
"what it would mean for a document to be true".
o Some on the WG do not believe that the WG is empowered
to make law;
that is a matter for the lawyers, governments, parliaments and the
like of
the many countries of the world. Different countries may make
different
laws.
They are right, RDF Core cannot determine the punishment to meted out
to
an individual who makes a false statement in given case and
a given jurisdiction. However, it is vital that RDFCore explain
concisely
and
unequivocally the algorithm for determining the meaning of an RDF
document
?? What algorithm for determining meaning? (Do you mean, algorithm for
checking validity of inference? We can do that.)
so that legal folks have a sound base for their own
arguments, but no
basis for wriggling out between the specs.
Note that RDF specs are referred to by XML specs (via the namespace)
which are referred to by the MIME registry which is referred to by the
HTTP
spec which is referred to by the TCP port registry, which is referred
to
by the TCP spec, which you effectively agree to when you get an IP
connection.
So there is a well accepted and important chain of delegation, in which
RDF plays a role of one link.
Until you get to RDF, this is all to do with computable operations
over computable domains, so has well-defined notions of meaning based
on fixed-point semantics. RDF is where it starts talking about the
wider world outside the recursive universe of computational processes,
and that is where 'meaning' suddenly gets far more difficult to pin
down.
No, it starts before RDF. An SMTP message, to TCP port 25 on an
internet host, saying "From: xxx" MEANS that there follows a message
from party xxx. This is a real world connection. The machinery
communicates my indent to address you with a message, and society will
hold me to having done that. The meaning of the message is normally
human-readable, but in the case of an OFX bank statement download, it
is machine processable and yet defined in english.
The universe as a whole does not satisfy the second
recursion theorem.
(See my www2002 keynote)
o Do you expect us to define exactly what an RDF
statement means?
_:b < < .
_:b < < .
Yes. By delegating to the specification of the Properties and Classes
used as predicate and (in the case of rdf:type predicate) object.
What chain of evidence would be required to prove that this
is a
derogatory statement about me.
The chain would be that I mentioned above showing that the
definitions of foo:Liar and foo:email define the meaning of the
document.
But there are no definitions in RDF.
The definitions are almost completely outside RDF. The DAML+OIL spec
for example has some definitions of DAML+OIL terms. They are
definitions in the sense of authoritative statements in that they are
made by the owners of the terms and published as such.
So this can only mean, at best, that there are some RDF
assertions
no some english
(in a graph somewhere) which are sufficient to constrain the
possible interpretations to those worlds in which Brian is, in fact, a
liar.
yes.
And that in turn requires that the concept of 'liar' is
somehow expressible in RDF, which is a pretty ambitious claim to make.
which is why i'm not making it
Suppose Brian sued me and I said in response that the term
'foo:Liar' was not intended to express the derogatory English meaning
of the English word "liar", and challenge him in response to show how
any conclusion that would be harmful to him could be derived as an
RDF-valid entailment from my RDF graph. Would that be a reasonable
defense, in your view? I would find it convincing, myself.
The conclusion that he is a liar is damaging. The foo spec states
that that is what the x rdf:type foo:Liar means.
To then show an HTTP response from server for foo,
which has
been, though established social procedures and technical specs of DNS
been
demonstrably
delegated the ability to publish information by the owner of the foo.
That response could contain information in a suitable language
What does 'suitable language' mean? Does it have to be suitable for
use by RDF reasoning agents?
No. There are more things in heaven and earth, Horatio, than can be
dreamed of in RDF.
No wonder folks have trouble layering DAML+OIL on top of RDF -- sounds
like a problem using RDF for any meaning at all.
which
indicated
Indicated to what/who? To a human reader, or to a software agent?
(Does the software agent only know RDF, or does it also know DAML or
OWL?)
that
foo:Liar was a class of people who were liars, and that foo:email was
an emailmox which a person used, then the chain would be complete
about the meaning of the document.
There is a simple step to show that no one else has your mailbox.
Now suppose that document were published by sending it to
a public email list. There is an indisputable body of history which
accepts that such a message is considered a public assertion by the
sender.
So you could sue.
Yes, but you would be suing because it was a public assertion *in
English* to *human readers*, which has got nothing at all to do with
RDF.
I am exactly saying that it has. While our logic may be nice for
defining numbers, it can't define and shoes and ships and sealing wax.
(Though no doubt some dream...)
The same slander case would probably work if RDF wasn't
involved anywhere, and you just had the character string 'Brian
McBride est un fichu menteur' on the web page. (Notice the use of
quotation in the previous sentence to mention, rather than use, a
character string.)
Yes.
You couldn't if the document were sent in an
attachment
as attachments break the chain, unless there is a something in the
cover
note
(e.g. "I agree with the enclosed") to connect the chain of assertion.
It is arguable what happens if it is published
on a website. Normally it is clear (for example though links from
someone's
home page) which documents published are deemed to be asserted, and
which
are archive copies of other people's things, for example.
Sure, but here you are talking about human assertions, right? That is,
assertions made in the context of human society and human discourse,
using human language. RDF isn't a human language; it's a new kind of
thing: a formalism for use by software agents which is also public in
a human world. I wonder if the courts are ready for this; I suspect
they will have to deal with a whole new set of issues that have never
arisen before.
They will have to deal with (1) the ownership of terms being rather
well defined, and less easy to argue about, particularly because if
you don't like foo:Liar you can go define pat:Liar in your own space;
and also with (2) some small aspect of terms being representable in
formal axioms. Here daml:transitiveProperty is a happy exception, as
its axioms leave nothing really to be said in addition. Or it will be
an exception when you get a complete axiomatic definition when you
look it up on the web.
These two changes will make automation and validation and stuff
simpler in a lot of ways. It still won't stop people arguing about
what "fair use" and "in good condition" mean.
But the MT only helps with (2). It is (1), the ability to introduce
new terms, which allows the use of RDF and the layering upon it.
Tim