of the word bird. When we talk about a bird in particular, well-designated, we are using the denotation of the word bird. Does that sound familiar? Of course it does. A particular bird is the content, just like the music of the song, and the generic bird is the description of that content. What Berners-Lee has done is merge together how humans work with content and how computers work with content. That’s the stuff of great progress! Examples of uniform resource identifiers are: “The sun,” “http://google.com” and “3.14159.” In short, any string of text can be made to describe something.

Now let’s get a little bit more specific. You’re going to say, fine, Berners-Lee just told me that we can mention Madonna’s song “Like a Virgin”, and that a computer can also use the terms “Like a Virgin”, and that the computer and I can understand each other. I can see that’s useful, even though I don’t quite see yet what’s so revolutionary about it. But that doesn’t advance me one bit in finding the song. On which computer can I find it? Where? So Berners-Lee also thought about that. We can associate any uniform resource identifier (say “Like a Virgin”, or “bird”) with the reference to any particular example of it (say the song on my computer’s hard disk, or the bird on my cousin’s Web site). And, to make things easier, I can use another invention of Berners-Lee, the Universal Resource Locator, to do so. Doesn’t that name ring a bell? Of course it does. It’s the famous URL: for example http://www.google.com is a universal resource locator, and so is http://mycousinwebsite.com/bird.jpg, a universal resource locator that points to the bird of my cousin’s Web site, and so is file:///D:/Songs/track1.jpg, which points to the recording of Madonna’s song in my laptop. In short, we now have a means to name everything on a computer, and more. Moreover, when we are talking about something on a computer, we can locate it easily.

The story doesn’t stop here. One more advance is needed. Now we can inquire of a computer about a song, such as “Like a Virgin” and about a singer, such as Madonna. We know how to find the song in a computer, and we know how to find the picture of Madonna on a computer; just point to a Web site with that picture. However, how does a computer associate the singer with the song? Can the computer say that “Like a Virgin” is a song by Madonna? In fact it can, and that’s where the Resource Description Framework comes into play. It is just a way to associate three universal description identifiers: “Like a Virgin,” “Song” and “Madonna.”

Again, we have to apologize, because computer speak is often not that friendly. However, we hope one can guess by looking at the sequence that we are talking about a song, whose singer is Madonna and whose title is Like a Virgin. If a computer is happy with that, so are we, because we’ve accomplished a huge feat! Of course, we have simplified somewhat, but not in ways that alter the general idea. Thus, there is a language that is explicit enough for computers and yet general enough for humans. Now, humans and computers can think along similar terms. How similar is what we are going to talk about next.

The second discipline that has affected a general theory of content is Artificial Intelligence. Here is the idea: can a computer contain knowledge and can a computer act on that knowledge in a way similar to humans? We will start by the question of knowledge. Previously, we have described how content can be organized into layers, with each layer containing a description of lower content, together with that content. Now, we will take the question from its more general perspective, and we will recognize that there is a domain of human science that has addressed it; that of logic. We’ve all gone through lengthy sessions at school saying things like x, y, z are

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