Knowledge Representation using other logic-semantic network

Semantic Network

A semantic net (or semantic network) is a knowledge representation technique used for propositional information. So it is also called a propositional net. Semantic nets convey meaning. They are two dimensional representations of knowledge. Mathematically a semantic net can be defined as a labelled directed graph.

Semantic nets consist of nodes, links (edges) and link labels. In the semantic network diagram, nodes appear as circles or ellipses or rectangles to represent objects such as physical objects, concepts or situations. Links appear as arrows to express the relationships between objects, and link labels specify particular relations. Relationships provide the basic structure for organizing knowledge. The objects and relations involved need not be so concrete. As nodes are associated with other nodes semantic nets are also referred to as associative nets.

 Consider the example,

• Tom is a cat.
• Tom caught a bird.
• Tom is owned by John.
• Tom is ginger in colour.
• Cats like cream.
• The cat sat on the mat.
• A cat is a mammal.
• A bird is an animal.
• All mammals are animals.
• Mammals have fur

 

 

It is argued that this form of representation is closer to the way humans structure knowledge by building  mental links between things than the predicate logic we considered earlier.Note In particular how all the information about a particular object is concentrated on the node

Representing that object, rather than scattered around several clauses in logic.There is, however, some confusion here which stems from the imprecise nature of  semantic nets.A particular problem is that we haven’t distinguished between nodes representing classes of  things,and nodes representing individual objects. So, for example, the node labelled Cat represents both the single (nameless) cat who sat on the mat, and the whole class of cats  to which Tom belongs, which are mammals and which like cream. The is_alink has two different meanings – it can mean that one object is an individual item from a class, for example Tom is a member of the class of cats,or that one class is a subset of another, for example, the class of cats is a subset of the class of mammals. This confusion does not occur in logic, where the use of quantifiers, names and predicates makes it clear what we mean so:

Tom is a catis represented by

Cat(Tom)

The cat sat on the matis represented by

∃x∃y(Cat(x)∧Mat(y)∧SatOn(x,y))

A cat is a mammal is represented by

∀x(Cat(X)→Mammal(x))

We can clean up the representation by distinguishing between nodes representing individualOr instances, and nodes representing classes. The is_a link  will only be used to show an Individual belonging to a class. The link representing one class being a subset of another will

be labelled a_kind_of , or  ako for short. The names instance and  subclass are  often  used  in the place of is_a and ako, but we will use these terms with a slightly different  meaning  in the section on Frames below.Note also the modification which causesthe  link labelled is_owned_by to be reversed in direction. This is in order to avoid  links

representing passive relationships. In general a passive sentence can be replaced by an active one, so “Tom is owned by John” becomes “John owns Tom”. In general the rule which converts passive to active in English converts sentences of the form “X is Yed by Z”  to  “Z Ys X”. This is just an example (though often used for illustration) of the much more general

principle of  looking beyond the immediate surface structure of a sentence to find its deep

structure.

 

 

Note that where we had an unnamed member of  some class, we have had to introduce a node with an invented name to represent a particular member of the class. This is a proces ssimilar to the Skolemisation we considered previously as a way of dealing with existential quantifiers. For example, “Tom caught a bird” would be represented in logic by ∃x(bird(x)∧caught(Tom,x))which would be Skolemised by replacing the x with a Skolem constant; the same thing was done above where bird1 was the name given to the individual bird that Tom caught.There are still plenty of  issues to be resolved if we really want to represent what is meant by the English phrases, or to be really clear about what the semantic net means, but we are getting towards a notation that can be used practically (one example of a thing we have skated over is how to deal with  mass nouns like “fur” or “cream” which refer to things that come in amounts rather than individual objects).