Semantic Networks

SEMANTIC NETS Semantic networks are knowledge representation schemes involving nodes and links (arcs or arrows) between nodes. The nodes represent objects or concepts and the links represent relations between nodes. The links are directed and labeled; thus, a semantic network is a directed graph

This network contains example of both isa and instance relations, as well as domain specific relations like team and uniform-color. In this network, we could use inheritance to derive the additional relation has-part(pee-Wee-Reese, Nose) 1. Intersection Search: Semantics nets were used to find relationships among objects

by spreading activation out from each of the two nodes and seeing where the activation met. This process is called Intersection Search. Using this process, it is possible to use the network of above figure to answer questions such as “What is the connection between Brooklyn-Dodgers and blue? 2. Representing Non-binary Predicates: Semantic nets are a natural way to represent

relationships that would appear as ground instances of binary predicates. Some of the arcs from the figure could be represented in logic as

isa(Person,Mammal) instance(Pee-Wee-Reese, Person) team(Pee-Wee-Reese,Brooklyn-Dodgers) uniform-color(Pee-Wee-Reese, Blue)

LIMITATIONS

1. Binary relations are usually easy to represent, but some times is difficult.E.g. try to

represent the sentence:"John caused trouble to the party".

2. Quantified statements are very hard for semantic nets. E.g.:  "Every dog has bitten a postman"  "Every dog has bitten every postman" Solution: Partitioned semantic networks can represent quantified statements. Partitioned semantic networks Hendrix developed the so-called partitioned semantic network to represent the difference between the description of an individual object or process and the description of a set of objects. The set description involves quantification. The central idea of partitioning is to allow groups, nodes and arcs to be bundled together into units called spaces Every node and every arc of a network belongs to (or lies in/on) one or more spaces. Some spaces are used to encode 'background information' or generic relations; others are used to deal with specifics called 'scratch' space. Suppose that we wish to make a specific statement about a dog, Danny, who has bitten a postman, Peter: "Danny the dog bit Peter the postman" Hendrix’s Partitioned network would express this statement as an ordinary semantic network:

The partitioning of a semantic network renders them more  logically adequate, in that one can distinguish between individuals and sets of

individuals, and  indirectly more heuristically adequate by way of controlling the search space

by delineating semantic networks. Hendrix's partitioned semantic networks-oriented formalism has been used in building natural language front-ends for data bases and for programs to deduct information from databases.

"John believes that pizza is tasty"

Every student loves to party"