Statements about statements#
Plain RDF says that Tim Berners-Lee worked at CERN. It has no comfortable way to say when that was true, who claims it, or that it was true twice over different periods (the statement itself is not something you can point at).
The 1.2 generation of the specifications fixes that. RDF 1.2 adds triple terms and reifiers, so a statement becomes a term that other statements can describe; SPARQL 1.2 adds the patterns to query them; and SHACL 1.2 adds the constraints to require and validate them.