AI Magazine Volume 24 Number 3 (2003) (© AAAI) Articles Where Are the Semantics in the Semantic Web? Michael Uschold ■ The most widely accepted defining feature of the ing primarily intended for human consump- semantic web is machine-usable content. By this tion to being intended for use both by humans definition, the semantic web is already manifest in and machines. shopping agents that automatically access and use Web tasks and services: The web is evolv- web content to find the lowest air fares or book ing from being primarily a place to find things prices. However, where are the semantics? Most to being a place to do things as well (Smith people regard the semantic web as a vision, not a 2001).1 reality—so shopping agents should not “count.” To use web content, machines need to know what All these new capabilities for the web de- to do when they encounter it, which, in turn, re- pend in a fundamental way on the idea of se- quires the machine to know what the content mantics, giving rise to another perspective means (that is, its semantics). The challenge of de- from which the web evolution can be viewed: veloping the semantic web is how to put this Semantics: The web is evolving from con- knowledge into the machine. The manner in taining information resources that have little which it is done is at the heart of the confusion or no explicit semantics to having a rich se- about the semantic web. The goal of this article is mantic infrastructure. to clear up some of this confusion. Despite the widespread use of the term se- I explain that shopping agents work in the com- mantic web, it does not yet exist except in iso- plete absence of any explicit account of the seman- lated environments, primarily in research labs. tics of web content because the meaning of the In the World Wide Web Consortium (W3C) Se- web content that the agents are expected to en- mantic Web Activity Statement, we are told counter can be determined by the human pro- that grammers who hardwire it into the web applica- tion software. I therefore regard shopping agents the Semantic Web is a vision: the idea of as a degenerate case of the semantic web. I note having data on the Web defined and various shortcomings of this approach. I conclude linked in a way that it can be used by ma- by presenting some ideas about how the semantic chines not just for display purposes, but web will likely evolve. for automation, integration and reuse of data across various applications (emphasis mine).2 he current evolution of the web can be As envisioned by Tim Berners-Lee: characterized from various perspectives (Jasper and Uschold 2003): the Semantic Web is an extension of the T current Web in which information is giv- Locating resources: The way people find things on the web is evolving from simple free en well-defined meaning, better enabling computers and people to work in cooper- text and keyword search to more sophisticated ation (Berners-Lee, Hendler, and Lassila semantic techniques both for search and navi- 2001, p. 35) (emphasis mine). gation. Users: Web resources are evolving from be- [S]omething has semantics when it can Copyright © 2003, American Association for Artificial Intelligence. All rights reserved. 0738-4602-2003 / $2.00 FALL 2003 25 Articles be ‘processed and understood by a com- The lack of an adequate definition of the se- puter,’ such as how a bill can be processed mantic web, however, is no reason to stop pur- by a package such as QUICKEN (Trippe suing its development any more than an inad- 2001, p. 1). equate definition of AI was a reason to cease AI There is no widespread agreement on exact- research. Quite the opposite, new ideas always ly what the semantic web is for or exactly what need an incubation period. it is. Some good ideas about what the semantic The research community, industrial partici- web will be used for have emerged from the pants, and software vendors are working with W3C effort to define a standard ontology lan- the W3C to make the semantic web vision a re- 6 guage.3 From the previous descriptions, there is ality (Berners-Lee et al 2001). It will be layered, clear emphasis on the information content of extensible, and composable. A major part will the web as machine usable and associated with entail representing and reasoning with seman- more meaning. tic metadata and providing semantic markup Note that machine refers to computers (or in the information resources. Fundamental to computer programs) that perform tasks on the the semantic infrastructure are ontologies, web. These programs are commonly referred to knowledge bases, and agents along with infer- as software agents, or softbots, and are found in ence, proof, and sophisticated semantic query- web applications. ing capability. Machine-usable content presumes that the The main intent of the semantic web is to machine knows what to do with information give machines much better access to informa- on the web. For this to happen, the machine tion resources so they can be information in- The web is reads and processes a machine-sensible specifi- termediaries in support of humans. According cation of the semantics of the information. to the vision described in Berners-Lee et al. evolving from This approach is robust and very challenging (2001), agents will be pervasive on the web, containing and largely beyond the current state of the art. carrying out a multitude of everyday tasks. information A much simpler alternative is for the human Hendler describes many of the important tech- web application developers to hardwire the nical issues that this approach entails, empha- resources that knowledge into the software so that when the sizing the interdependence of agent technolo- have little or machine runs the software, it does the correct gy and ontologies (Hendler 2001). To carry out thing with the information. In this second sit- their required tasks, intelligent agents must no explicit uation, machines already use information on communicate and understand meaning. They semantics to the web. There are electronic broker agents in must advertise their capabilities and recognize routine use that make use of the meaning asso- the capabilities of other agents. They must lo- having a rich ciated with web content words, such as price, cate meaningful information resources on the semantic weight, destination, and airport. Armed with a web and combine them in meaningful ways to infrastructure. built-in understanding of these terms, these so- perform tasks. They need to recognize, inter- called shopping agents automatically peruse pret, and respond to communication acts from the web to find sites with the lowest price for a other agents. book or the lowest airfare between two given In other words, when agents communicate cities. Thus, we still lack an adequate character- with each other, there needs to be some way to ization of what distinguishes the future seman- ensure that the meaning of what one agent tic web from what exists today. “says” is accurately conveyed to the other Because the RESOURCE DESCRIPTION FRAMEWORK) agent. There are two extremes, in principle, for (RDF) is hailed by the W3C as a semantic web handling this problem. The simplest (and per- language,4 some people seem to have the view haps the most common) approach is to ignore that if an application uses RDF, then it is a se- the problem altogether. That is, just assume mantic web application. This is reminiscent of that all agents are using the same terms to the “if it is programmed in Lisp or Prolog, then mean the same things. In practice, this as- it must be AI” sentiment that was sometimes sumption will usually be built into the appli- evident in the early days of AI. There is also cation. The assumption could be implicit and confusion about what constitutes a legitimate informal, or it could be an explicit agreement semantic web application. Some seem to have among all parties to commit to using the same the view that an RDF tool such as CWM is one.5 terms in a predefined manner. This approach This is true only in the same sense that KEE and only works, however, when one has full con- ART were AI applications. They were certainly trol over what agents exist and what they generating income for the vendors, which is might communicate. In reality, agents need to different from the companies using the tools to interact in a much wider world, where it can- develop applications that help their bottom not be assumed that other agents will use the line. same terms, or if they do, it cannot be as- 26 AI MAGAZINE Articles sumed that the terms will mean the same have semantics, where the semantics are, and thing. how they are used. We identify a kind of se- The moment one accepts the problem and mantic continuum ranging from the kind of grants that agents might not use the same semantics that exist on the web today to a rich terms to mean the same things, one needs a semantic infrastructure on the semantic web of way for an agent to discover what another the future. agent means when it communicates. Thus, Real-world semantics: Real-world seman- every agent needs to publicly declare exactly tics7 are concerned with the “mapping of ob- what terms it is using and what the terms jects in the model or computational world on- mean. This specification is commonly referred to the real world … [and] issues that involve to as the agent’s ontology (Gruber 1993).