Metadata, Resources, and the Resource Description Framework 7 A set of documents which has a set of properties can be described as nodes in a network, with arrows (properties) pointing at the values of the properties. Since each property applies to only one object, this is a directed graph, a dia- gram that you can follow in one direction only.  The arrows do not have to go in one direction (but there has to be one arrow per direction). For a simple graph, this is not complicated. Here is what the statement,  Napoleon was emperor, looks like as a directed graph: Napoleon emperor (As a matter of fact, I am cheating. What it says is  Napoleon has the property Emperor.   It  says  nothing  about  the  temporal  aspect,  that  he  actually  is  no longer  emperor).  If  you  are  familiar  with  object-oriented  programming,  you will see that this is the same as the Booch methodology analysis statement that the object Napoleon has the property Emperor. And indeed, RDF is a format to describe  the  properties  of  objects,  which  are  identified  by  their  Uniform Resource Identifiers (URIs). You do not, however, have to subscribe to any of the  mysticism  often  surrounding  object-oriented  programming.  The  objects here are not programming objects, they are information objects, which is dif- ferent.  Information  objects  do  not  follow  the  same  rules  as  programming objects. For instance, they cannot do anything (they do not have methods asso- ciated with them), and the inheritance of properties between classes is not at all as clear-cut. RDF is primarily intended to describe information objects, but it is not really restricted to that. It can be used to describe anything that has a URI, which in principle can be any object. And because objects are related to our perception ( What constitutes an object?  is a question that we will not go into here, but it is one that philosophers struggle with), anything we can perceive or imagine can be an object. And provided it can be given an identifier, it can be described mathematically. Provided that identifier can be a URI, it can be described in RDF, as it can then be a resource in the sense of RDF. The  graph  above  can  also  be  formulated  as  a  triple  of  values:  The  resource being  described,  the  property  name,  and  the  property  value.  RDF  describes how to take the triples into XML, and how to do this in a way that maintains the integrity of the graph. The W3C runs an excellent validation service based on the Simple RDF Parser and Compiler (SiRPAC). Figure 1.1 is an example of what the graph would look like, as well as the RDF associated with it. To a mathematician, the paper drawing and the mathematical formulation of the graph are the same thing, just formatted a little differently. This means that to  a  mathematician,  an  XML  encoding  of  the  graph  is  just  another  form  of expressing it. You can interchange the different description formats; they are 69528_CH01Ix  4/6/2001 8:15 AM  Page 7