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What is so semantic in Semantic web anyway?

Filed under: Philosophy — Tags: , — Davor @ 11:09

Semantic web. Semantic? This eluded me back from the time I first heard the term. It is concerned with the meaning and not as much with the structure of data. But how?

The term “semantic” was coined by Tim Berners-Lee for a web of data that can be processed by machines. But that doesn’t tell us why it is called semantic. What has meaning to do with machines and processing?

After I saw this MIT presentation on the suject it dawned upon me that there is an interesting equivalence between how truth is defined in Semantic web, and the way Donald Davidson defined meaning in his “theory of meaning”. His ideas go back to the 60s, and are based on Tarski’s theory of truth from the 30s. So here is my colloquized way of explaining the intuition of why the Semantic web is actually semantic.

The semantinc web in the MIT presentation is defined as:

XML + RDF + Ontologies + Inference rules = Semantic web!

You wonder why all this equates to “Semantic”?

Suppose you have a set of entities, lets say {Socrates, man, mortal}. Suppose don’t know about nothing else than those three words. Suppose that that is your Ontology. That is your world. Note that having an Ontology is a constitutive requirement for a Semantic web. So are the Inference rules. Suppose we have only one Inference rule: if A is B, and B implies C then A is C. The third constitutive component (RDF) consists of  some true statements about your set of objects which are contained in the subjectpredicateobject structure. For example: Socrates (subject) is (predicate) a man (object) and men (subject) are (predicate) mortal (object). XML is used to describe all of the other three so a machine can read and interpret them. Now given the preceding example, what the machine can do is deduce the truth of a statement like: Socrates is a man, all men are mortal, therefore Socrates is mortal (implication from Inference rule). Now remember (!), given its Ontology, Inference rules and RDF, the machine knows which statement is true, and which is not true. The machine knows that the statement Socrates is mortal is true!

Now, where is semantics in all this? Well, Davidson proposed the idea that truth and meaning are equivalent. If you have one, then you have the other too. Note that given the RDFs, Ontology and Inference rules, the machine actually knows when any statement is true. Thus the machine knows the meaning of the statement.

That too quick? Not convinced? Well, suppose I ask you whether the following statement is true: “Socrates je covjek”. Can you assess the truth? Not if you don’t know Croatian – which I assume you do not for the sake of the example. But suppose I tell you that the statement “Socrates je covjek” is true under all conditions under which “Socrates is a man” is true (i.e. the two statements are equivalent). Since you know that “Socrates is a man” is true (i.e. it is explicitly stated in your RDF), and that “Socrates je covjek” is true whenever “Socrates is a man” is true, then you can perfectly say that you understand “Socrates je covjek” and thus know its meaning. In other words, if you know the truth conditions of “Socrates je covjek”, you understand it. The same can be done with “Socrates is mortal”. Although it is not explicitly stated in the RDF, the machine can deduce its truth. So if “Chapa muju koki” is equivalent to “Socrates in mortal”, then you can say that you understand it. That is the mechanism behind the Semantic web. Knowing the truth of a statement implies knowing the meaning of the statement – and vice versa.

Thus the web is semantic.

Now you can ask whether meaning is not more than only truth. That is a difficult question for which I can not go into much details here. My philosophical days are unfortunately numbered. But a good starting point on this subject is probably here. My own intuition is that meaning as defined above is only a subset of what we understand under meaning. Thus, the web is semantic, but only up to a certain degree…

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