\subsection{Authorities, authorization and warrants}
\label{sec:5.1}

The formal semantics of RDF establishes truth-conditions, but it does 
not provide for any explicit way for an information provider to 
express any intention concerning the truth-value of the information; 
it does not provide for the expression of \emph{propositional 
attitudes}. Information consumers may require this, however. 
\todo{can we say something stronger than this? eg cite loud demands 
for it on rdf-comments?} Note that this is in addition to trust 
policies, and may be required in order to put such policies into 
operation. For example a simple policy could be: believe anything 
asserted by a trusted source. In order to apply this, it is necessary 
to have a clear record of what is \emph{asserted} by the source. Not 
all information provided by a source need be asserted by that source. 
We propose here a vocabulary and a set of concepts designed to enable 
the uniform expression of such propositional attitudes using named 
graphs.

We take three basic ideas as primitive: that of an \emph{authority}, 
a relationship of \emph{authorizing}, and a \emph{warrant}. An 
authority is a 'legal person'; that is, any legal or social entity 
which can perform acts, undertake obligations and is subject to laws 
and social norms and expectations. Examples include adult humans, 
corporations and governments. The 'authorizing' relationship holds 
between an authority or authorities and a named graph, and means that 
the authority in some sense stands behind, or commits itself to, the 
content expressed in the graph. Whether or not this relationship in 
fact holds may depend on many factors and may be detected in several 
ways. We suggest however that two obvious candidates are when a named 
graph is published on a website owned by an authority, and when a 
named graph is digitally signed by an authority. Finally, a warrant 
is a resource which records a particular propositional stance or 
intention of an authority towards a graph. A warrant asserts (or 
denies or quotes) a named graph and is authorized by an authority. 
One can think of warrants as a way of reducing the multitude of 
possible relationships between authorities and graphs to a single one 
of authorization, and also as a way of separating questions of 
propositional attitude from issues of checking and recording 
authorizations.  The separation of authority from intention also 
allows a single warrant to refer to several graphs, and for a warrant 
to record other properties such as publication or expiry date.

To describe the two aspects of a warrant we require vocabulary items: 
a property \swp{authority} (where swp: is a namespace for Semantic 
Web publishing) relating warrants to authorities, and another to 
describe the attitude of the authority to the graph being represented 
by the warrant. We will consider two such intentions expressed by the 
properties \swp{assertedBy} and \swp{quotedBy}. These take a named 
graph as a subject and an \swp{Warrant} as object; \swp{authority} 
takes a warrant as a subject and an \swp{Authority} as an object. 
Each warrant must have a unique authority, so swp:authority is an OWL 
functional property.  Intuitively, \swp{assertedBy} means that the 
warrant records an endorsement or assertion that the graph is true, 
while \swp{quotedBy}  means that the graph is being presented without 
any comment being made on its truth. This latter is particularly 
useful when republishing graphs as part of a syndication process, the 
original publisher may assert a news article, but the syndicator, 
acting as a common carrier, merely provides the graph as they found 
it, without making any commitment as to its truth. Warrants may also 
be signed, and the vocabulary \swp{signatureMethod} property can be 
used to identify the signature technique.

\todo{<<< insert figure 1 near here >>>}
\subsection{Warrant 
descriptions as performatives}
\label{section 5.2}

Consider a graph 
containing a description of a warrant of another named graph, such as:

\begin{rdfenv}
\{ :G2 swp:assertedBy :G1 .
:G1 rdf:type swp:Warrant .
:G1 swp:authority _:a .
_:a rdf:type swp:Authority .
_:a foaf:mbox <mailto:chris@bizer.de> \}
\end{rdfenv}

Call this a \emph{warrant graph}.  The graph is true when G1 denotes 
a genuine warrant; but so far we have no way to know whether this is 
in fact the case.  Now however suppose that this graph is G1 itself:

\begin{rdfenv}
:G1  \{ :G2 swp:assertedBy :G1 .
:G1 rdf:type swp:Warrant .
:G1 swp:authority _:a .
_:a rdf:type swp:Authority .
_:a foaf:mbox <mailto:chris@bizer.de> \}
\end{rdfenv}

then the graph describes itself as being a warrant. Suppose further 
that this graph is in fact authorized by the authority it describes - 
in this case, by Christian Bizier, the owner of the mailbox: this 
might be established for example by being published on Christian's 
website, or by being digitally signed by him, or in some other way, 
but all that we require here is that it is in fact true. Under these 
circumstances, the warrant graph has the intuitive force of a 
first-person statement to the effect "I assert G2" made by Christian.

In natural language, the utterance of such a self-describing act is 
called a \emph{performative}; that is, an act which is performed by 
saying that one is doing it. Other examples of performatives include 
promising, naming and, in some cultures, marrying [2]. The key point 
about performatives are that while they are descriptions of 
themselves, they are not only descriptions: rather, the act of 
uttering the performative is understood to be the act that it 
describes. Our central proposal for how to express propositional 
attitudes on the Web is to treat a self-describing warrant graph as a 
record of a performative act, in just this way <<footnote 7>>. With 
this convention, Christian Bizier can actually, in fact, assert the 
graph G2 by authorizing the warrant graph shown above, for by doing 
so he creates a warrant: the warrant graph becomes the 
(self-describing) warrant of the assertion of G2 by Christian. In 
order for others to detect and confirm the truth of this warrant 
requires some way to check or confirm the relationship of 
authorization, of course: but the qualification of the warrant graph 
as a warrant depends only on the relationship holding.

A warrant graph is not required to be self-describing in order to be 
true (it may be true by virtue of some other warrant) and a 
self-describing warrant graph may not in fact be a performative 
warrant (if it is not authorized by the authority it claims). In the 
latter case the graph must be false, so self-describing warrant 
graphs whose authorization cannot be checked should be treated with 
caution.  The warrant graph may itself be the graph asserted. Any 
named graph which has a self-describing warrant graph as a subgraph 
and is appropriately authorized satisfies the conditions for being a 
performative warrant of itself. For example:


\begin{rdfenv}
:G2 \{ :Monica ex:name "Monica Murphy" .
        :G2 swp:assertedBy :G2 .
        :G2 swp:authority _:a .
        _:a foaf:mbox mailto:patrick.stickler@nokia.com> .
\}
\end{rdfenv}

when authorized by Patrick Stickler, becomes a performative warrant 
for its own assertion, as well as being warranted by the earlier 
example. As this example indicates, a named graph may have a number 
of independent warrants.

These conventions are described more formally in section 6 below.


