Suppose person A tells person B that Clyde is an elephant. Person B ought immediately to be able to infer the following facts (and probably a lot more besides). (1) Clyde has a trunk. Clyde is grey. Clyde has thick skin. What has allowed multiple inferences to be drawn like this is a rule of inference which allows anything which is true of elephants also to be true of Clyde, since Clyde is an elephant. In fact, this description of the rule is too simple as it stands. Suppose person A tells person B that Clyde is a pink elephant. The rule of inference must be revised to state that everything which is true of elephants is also true of Clyde, except for those facts about elephants which are blocked (that is, contradicted) by known facts about Clyde. The first version of the rule is commonly referred to as "inheritance"; Clyde is said to inherit properties from elephant. The second version of the rule is called "default inheritance"; Clyde inherits properties from elephant only if those properties are not already specified for Clyde. [...] "Default inheritance". If X and Y are nodes, X may inherit from Y if a fact identifying Y as an inheritance source is included at X. All attribute: value pairs at Y become available at X, except those having an attribute which is already present in an attribute: value pair at X.
- Corbett & Fraser (2004), a pag.369