the rationale and history of RDF-star semantics

Dear Enrico, all,

following the discussion we had during the last call [1], I would like 
to give a more detail overview of our rationale when designing the 
RDF-star semantics in the CG report [2], and defuse a few misunderstandings.

1. It was not the intention to make RDF-star a modal logic

I know that examples such that ":alice :belives << :s :p :o >>." 
strongly point in the direction of modal logic, and that such examples 
have been largely used to "sell" RDF-star. I agree that such examples 
are misleading, and we actually tried to avoid such examples in the CG 
report.

The intention was to make RDF-star quoted triples opaque, and providing 
as little inferences as possible -- leaving it open for semantic 
extensions to provide more inferences.


2. Ground quoted triples are similar to literals

Our initial attempt was to define from scratch a model theoretic 
semantics of RDF-star, where ground quoted triples (i.e. quoted triples 
containing no blank node) were constrained to denoted themselves. In 
other words, we consider that RDF(-star) triples (as defined by the 
specification) are conceptual objects that exist in the world (in the 
same way that graphs, classes and properties exist), and that ground 
quoted triples did denote exactly them.

In that sense, ground quoted triples are very much like literals (except 
that they are allowed in the subject position).


3. Blank node rain on our parade (as they usually do)

Of course, things get tricky when we take blank nodes into account.

Just like the RDF1.1 Semantics, our proposal was built in two steps :
- define the semantics of ground RFD-star graphs (following the 
rationale described above)
- deal with blank node

The second step was quite complicated, and it raised some questions 
about whether this brand new semantics was sound. An alternative way was 
therefore proposed, to rely on the battle-tested RDF semantics. And 
that's where we are now.

Honestly, I would rather give another try at /adapting /the current RDF 
semantics to take into account quoted graphs, than keeping the layered 
approach that we currently have.

   pa


[1] https://www.w3.org/2023/01/12-rdf-star-minutes.html#t03

[2] https://www.w3.org/2021/12/rdf-star.html#rdf-star-semantics