\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 illustrates, a named graph may have a number of independent warrants.

These conventions are described more formally in section 6 below.

