Identity, Reference, and Meaning on the Web

Problems of reference, identity, and meaning are becoming increasingly endemic on the Web. We focus first on the convergence between Web architecture and classical problems in philosophy, leading to the advent of “philosophical engineering.” We survey how the Semantic Web initiative in particular provoked an “identity crisis” for the Web due to its use of URIs for both “things” and web pages and the W3C’s proposed solution. The problem of reference is inspected in relation to both the direct object theory of reference of Russell and the causal theory of reference of Kripke, and the proposed standards of new URN spaces and Published Subjects. Then we progress onto the problem of meaning in light of the Fregean slogan of the priority of truth over reference and the notion of logical interpretation. The popular notion of “social meaning” and the practice of tagging as a possible solution is analyzed in light of the ideas of Lewis on convention. Finally, we conclude that a full notion of meaning, identity, and reference may be possible, but that it is an open problem whether or not practical implementations and standards can be created. 1. PHILOSOPHICAL ENGINEERING While the Web epitomizes the beginning of a new digital era, it has also caused an untimely return of philosophical issues in identify, reference, and meaning. These questions are thought of as a “black hole” that has long puzzled philosophers and logicians. Up until now, there has been little incentive outside academic philosophy to solve these issues in any practical manner. Could there be any connection between the fast-paced world of the Web and philosophers who dwell upon unsolvable questions? In a surprising move, the next stage in the development of the Web seems to be signalling a return to the very same questions of identity, reference, and meaning that have troubled philosophers for so long. While the hypertext Web has skirted around these questions, attempts at increasing the scope of the Web can not: “The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation” [41]. Meaning is a thorny word: do we define meaning as “machine-readable” or “has a relation to a formal model?” Or do we define meaning as “easily understood by humans,” or “somehow connected to the world in a roCopyright is held by the author/owner(s). WWW2006, May 22–26, 2006, Edinburgh, UK. . bust manner?” Further progress in creating both satisfying and pragmatic solutions to these problems in the context of the Web is possible since currently many of these questions are left underspecified by current Web standards. While many in philosophy seem to be willing to hedge their bets in various ideological camps, on the Web there is a powerful urge to co-operate. There is a distinct difference between the classical posing of these questions in philosophy and these questions in the context of the Web, since the Web is a human artifact. The inventor of the Web, Tim Berners-Lee, summarized this position: “We are not analyzing a world, we are building it. We are not experimental philosophers, we are philosophical engineers” [2]. 2. THE IDENTITY CRISIS OF URIS The first step in the creation of the Semantic Web was to extend the use of a URI (Uniform Resource Identifier) to identify not just web pages, but anything. This was historically always part of Berners-Lee’s vision, but only recently came to light with Semantic Web standardization efforts and has caused disagreement from some of the other original Web architects like Larry Masinter, co-author of the URI standard [4]. In contrast to past practice that generally used URIs for web pages, URIs could be given to things traditionally thought of as “not on the Web” such as concepts and people. The guiding example is that instead of just visiting Tim Berners-Lee’s web page to retrieve a representation of Tim Berners-Lee via http, you could use the Semantic Web to make statements about Tim himself, such as where he works or the color of his hair. Early proposals made a chasm across URIs, dividing them into URLs and URNs. URIs for web pages (documents) are URLs (Uniform Resource Locators) that could use a scheme such as http to perform a “variety of operations” on a resource[5]. In contrast, URNs (Uniform Resource Names) purposely avoided such access mechanisms in order to create “persistent, location-independent, resource identifiers” [29]. URNs were not widely adopted, perhaps due to their centralized nature that required explicitly registering them with IANA. In response, URLs were just called “URIs” and used not only for web pages, but for things not on the web. Separate URN standards such as Masinter’s tdb URN space have been declared, but have not been widely adopted [27]. Instead, people use http in general to identify both web pages and things. There is one sensible solution to get a separate URI for the thing if one has a URI that currently serves a representation of a thing, but one wishes to make statements about the thing itself. First, one can use URI redirection for a URI about a thing and then resolve the redirection to a more informative web page [28]. The quickest way to do this is to append a “hash” (fragment identifier) onto the end of a URI, and so the redirection happens automatically. This is arguably an abuse of fragment identifiers which were originally meant for client-side processing. Yet according to the W3C, using a fragment identifier technically also identifies a separate and distinct “secondary resource” [23]. Regardless, this ability to talk about anything with URIs leads to a few practical questions: Can I make a statement on the Semantic Web about Tim Berners-Lee by making a statement about his home-page? If he is using a separate URI for himself, should he deliver a representation of himself? However, in all these cases there is the lurking threat of ambiguity: There is no principled way to distinguish a URI for a web page versus a URI for a thing “not on the Web.” This was dubbed the Identity Crisis, and has spawned endless discussions ever since [9]. For web pages (or “documents”) it’s pretty easy to tell what a URI identifies: The URI identifies the stream of bits that one gets when one accesses the URI with whatever operations are allowed by the scheme of the URI. Therefore, unlike names in natural language, URIs often imply the potential possession of whatever representations the URI gives one access to, and in a Wittgenstein-like move David Booth declares that there is only a myth of identity [7]. What a URI identifies or means is a question of use. “The problem of URI identity is the problem of locating appropriate descriptive information about the associated resource – descriptive information that enables you to make use of that URI in a particular application” [7]. In general, this should be the minimal amount of information one can get away with to make sure that the URI is used properly in a particular application. However, if the meaning of a URI is its use, then this use can easily change between applications, and nothing about the meaning (use) of a URI should be assumed to be invariant across applications. While this is a utilitarian and attractive reading, it prevents the one thing the Web is supposed to allow: a universal information space. Since the elementary building blocks of this space, URIs, are meaningless without the concrete context of an application, and each applications may have orthogonal contexts, there is no way an application can share its use of URIs in general with other applications. 3. URIS IDENTIFY ONE THING Tim Berners-Lee has stated that URIs “identify one thing” [3]. This thing is a resource. The most current IETF RFC for URIs states that it does not “limit the scope of what might be a resource” but that a resource “is used in a general sense for whatever might be identified by a URI” such as “human beings, corporations, and bound books in a library” and even “abstract concepts” [4]. An earlier RFC tried to ground out the concept of a resource as “the conceptual mapping to an entity or set of entities, not necessarily the entity which corresponds to that mapping at any particular instance in time” in order to deal with changes in particular representations over time, as exemplified by the web sites of newspapers like http://www.guardian.co.uk [4]. If a URI identifies a conceptual mapping, in whose head in that conceptual mapping? The user of the URI, the owner of the URI, or some common understanding? While Tim Berners-Lee argues that the URI owner dictates what thing the URI identifies, Larry Masinter believes the user should be the final authority, and calls this “the power of readers over writers.” Yet this psychological middle-man is dropped in the latest RFC that states that URIs provide “a simple and extensible means for identifying a resource,” and a resource is “whatever might be identified by a URI” [4]. In this manner, a resource and its URI become disturbing close to a tautology. Given a URI, what does it identify? A resource. What’s a resource? It’s what the URI identifies. According to Berners-Lee, in a given RDF statement, a URI should identify one resource. Furthermore, this URI identifies one thing in a “global context” [2]. This position taken to an extreme leads to problems: given two textually distinct URIs, is it possible they could identify the same thing? How can we judge if they identify the same thing? The classic definition of identity is whether or not two objects are in fact, on some given level, the same. The classic formulation is Leibniz’s Law, which states if two objects have all their properties in common, then they are identical and so only one object [25]. With web pages, one can compare the representations byte-by-byte even if the URIs are different, and so we can say two mirrors of a web pages have the sa