Recently the term “Semantic Web” becomes extremely popular that Sitepoint blogs keep posting articles on this topic (1, 2). In my college days, I learned about Semantic Network and I wonder if there is some relationship between them. I’m not sure whether I get the concept correctly but in this article I would like to revise a bit on semantic network before going to semantic web. Please correct me if I’m wrong.
What is Semantic Network
As far as I know, a semantic network is a type of knowledge representation. A mind map, IMO, is a type of network diagram which loosely based on semantic network. Back to the topic, a semantic network is usually drawn to represent some knowledge through interconnecting nodes with labeled arcs. One of the objectives in drawing a semantic network is to do inferencing by referring to the interconnecting nodes. (I will post diagram once I find my e-copy of the lecture slides to illustrate)
Because my lecturer discussed about Prolog before moving on to semantic network, so we discuss semantic network in conjunction with Prolog way to represent facts. The steps taken to produce a semantic network for the fact “Mary is a lecturer” may be as follows:
- Change the fact into Prolog fact:
- Rewrite the prolog fact into another form:
- Now draw a semantic network with two nodes – mary and lecturer.
- Then connect the two nodes with an arc labeled instance (diagrams coming soon)
Of course the above produced network is simple because the given fact is simple in the first place. However, when there are more than two arities (predicates) in the prolog fact then the semantic network can be reasonably complex as follows.
My lecturer discussed the inferencing process with an example that uses multiple related facts. For example, we have two person, Peter and Jane each has the height of 180cm and 177cm. The initial semantic network may seems as follows:
To do inferencing, a special procedure is added to the network to process the nodes. Without the special procedure then the analysis of the diagram would become limited.
As you can see the semantic network got expanded and an arc is added to link the previously unrelated facts. Hence, we can make an inference through observing the diagram that Peter has the height of 180cm greater than Jane’s at 177cm.
Then the semantic network can be expanded as complex as possible in anyway one may think logical. Of course there may be more to cover in this topic like expanding and partitioning the semantic network, using frames to overcome the limitations of semantic network etc. But I would just stop here to check whether I have made any mistake before moving on.
Further reading: Semantic Network by John F. Sowa