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Re: Subjects as Literals

From: Sampo Syreeni <decoy@iki.fi>
Date: Mon, 5 Jul 2010 21:36:56 +0300 (EEST)
To: Hugh Glaser <hg@ecs.soton.ac.uk>
cc: David Booth <david@dbooth.org>, Kingsley Idehen <kidehen@openlinksw.com>, "nathan@webr3.org" <nathan@webr3.org>, Pat Hayes <phayes@ihmc.us>, Toby Inkster <tai@g5n.co.uk>, Dan Brickley <danbri@danbri.org>, Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>
Message-ID: <Pine.LNX.4.64.1007052116520.8363@lakka.kapsi.fi>
On 2010-06-30, Hugh Glaser wrote:

> RDF permits anyone to say anything about anything . . . except a 
> literal if it is the subject of the property you want to use for the 
> description.

The way I see it, the main reason for this restriction is that the data 
is supposed to be machine processable. Literals rarely are, especially 
in their original, plain form. I mean, suppose we allowed literals as 
subject, predicate and object. What does it really mean if I say 
"Sally"@en "likes"@en "Mike"@en?

I'd argue pretty much nothing processable. That's because literals tack 
on an arbitrary, limited number of type specifiers (type and perhaps 
language) to textual data, and neglect everything beyond that. That is 
not how full disambiguation is done; it's how a human processor 
*minimally* disambiguates a piece of text, without making it 
unambiguous.

With Schema derived or otherwise strictly derived types, the level of 
disambiguation can be the same as or even better than with URI's, true. 
But then that goes the other way around, too: URI's could take the place 
of any such precise type. Beyond that, all that literals do is invite 
people to import more ambiguity into the RDF/SemWeb framework.

So, better to limit that to the object, in case we just *have* to have 
it somewhere. (I'd rather do without entirely.)

> So I can say: foo:booth isNamed "David Booth" But of course I can't 
> say: "David Booth" isNameOf foo:booth

You can say the same thing, as you pointed out. So you're aiming at 
grammatical symmetry in excess of expressive capability. Why? There is 
definite value in making the relation asymmetric: that way you can be 
surer that what is being talked about stays...the subject. It's not by 
any means sure that that is really going to be useful, no.

But at the same time it's perfectly sure that you would have to start 
employing triples with both the subject and the object a literal, before 
the current model can constrain you semantically. That'd then be pretty 
extreme: the precise semantics of literals are tricky enough as they 
stand. Pretty much the only genuine use cases I can come up with 
off-hand are explicit unit conversions and label translations -- and 
then anything that goes that far should probably get URI's and/or 
epi-RDF conversion code in the first place. After all, both scenarios 
also call for context, which might have to be disambiguated beyond 
mere lexical form and type. (E.g. homologues or units with identical 
dimensions but different usage.)
-- 
Sampo Syreeni, aka decoy - decoy@iki.fi, http://decoy.iki.fi/front
+358-50-5756111, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
Received on Monday, 5 July 2010 18:37:59 UTC

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