主要解决的问题：
explore how we can represent a domain specific knowledge using the different concepts.
这篇文章：
we will see how we can represent the concepts and how we can represent the knowledge using these concepts.
We cannot call any collection of facts or information to be knowledge unless we can user it for inferring some new facts.
So if we to call this semantic net as a knowledge representation method or scheme ,then we must also have some particular inference mechanism by which we can utilize this representation to infer new things to answer different questions.
cognitive economy: how much we store.
Tests on human subjects showed that the subjects recognise propositions lower down the hierarchy more readily than propositions higher up the hierarchy.
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A semantic network is a structure for representing knowledge as a pattern of interconnected nodes and arcs.Nodes in the net represent concepts of entities, attributes, events, values. Arcs in the network represent relationships that hold between the concepts.
还有需要注意的是：
The semantics lies not in the structure alone,but also requires the relations.
(比如：一个arcs经常表示的relation是“is-a”，但若把这种relation换成“eat”，那么，一样的network structure会呈现不同的意义）