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Some questions on RDF 1.1 Reification Semantics

From: thomas lörtsch <tl@rat.io>
Date: Fri, 13 Jul 2018 21:49:38 +0200
Message-Id: <9EA353D6-B04C-41C1-8932-23A2F265B3A9@rat.io>
To: semantic-web@w3.org
I’m trying to understand what the RDF 1.1 Semantics Recommendation says about reification (*) but I’m having particular difficulties keeping up with the different kinds of triples it describes.

At one point quite early in Appendix D.1 the Recommendation says: 
"Reification is not a form of quotation. Rather, the reification describes the relationship between a token of a triple and the resources that the triple refers to."
I’m not a native speaker so some subtleties are lost with me. My best guess is that "token" here is meant as in type-token-distinction as at a later point the spec refers to "a particular instance or token of a triple". However if the spec refers to token as in type-token then why is the reification not describing the type but the token? Or would the spec, because it is (I guess) referring to unstated triples here, rather speak about (non-existing) instances than about their type? And if some triple with a specific subject/predicate/object is foremost a type how does that fit with the set semantics that there can be only one instance of that type? In my intuition it doesn’t. The dictionary also offers "symbol" and "representation" which can mean type or instance, so that doesn’t help either.

Shortly thereafter:
"Reifications can be written with a blank node as subject, or with an IRI subject which does not identify any concrete realization of a triple, in both of which cases they simply assert the existence of the described triple."
What is a "concrete realization"? The next sentence more specifically speaks of "a concrete realization of an RDF triple, such as a document in a surface syntax" but does that exclude triples in databases, or only unstated triples?

In the next sentence: 
"The subject of a reification is intended to refer to a concrete realization of an RDF triple, such as a document in a surface syntax, rather than a triple considered as an abstract object."
What is a triple as an "abstract object": a triple that merely exists (in the sense that it has never actually been stated)? Or a triple that sits in a database as bits and bytes but not "concretely realized"? Or both? Or anything but a triple that has been "concretely realized"?

To sum up, there seem to exist:
- abstract triples
- concretely realized triples
- simply existing triples
- reified/described (but not quoted) tokens (of triples)
Which of them has actually be asserted somewhere, somehow. Does it make a difference how "concertely" it has been "realized" - in a database, serialized to a turtle document, etc? Why does reification refer to a token, not the type? What is that token exactly?

It might be more useful if the spec differntiated just between things that can be asserted and things that actually have been asserted. Then some triple with a specific subject/predicate/object exists only once as something assertable, but it can be asserted many times (and each time may have an identifier, provenance etc). 

But back to one last question:
"[…] asserting a triple does not automatically imply that any triple tokens exist in the universe being described by the triple. For example, the triple might be part of an ontology describing animals, which could be satisfied by an interpretation in which the universe contained only animals, and in which a reification of it was therefore false."
That doesn’t look like anything to me… 
Does this suggest that a triple token is the real world realization of whatever the triple is refering to? I hope not, but I’m lost here anyway.

Thomas Lörtsch

(*) not because I want to use it but because I want to precisely understand its semantics (or lack thereof)
Received on Friday, 13 July 2018 19:51:43 UTC

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