A semantic network encodes that 'Bird can-fly' and 'Penguin is-a Bird.' A student adds a 'Penguin cannot-fly' property directly to the Penguin node. Which statement best describes how the network handles this exception?
AThe network raises a logical contradiction and cannot represent both facts simultaneously
BThe more specific (closer) property on Penguin overrides the inherited property from Bird, so Penguin cannot fly
CThe network ignores the exception because inherited properties always take precedence
DThe network requires the student to delete the 'Bird can-fly' edge before adding the exception
Semantic networks handle exceptions through specificity: more specific (closer in the inheritance hierarchy) properties override general inherited ones. The Penguin node's direct 'cannot-fly' property takes precedence over the 'can-fly' property inherited via the Bird is-a path. This is what makes semantic networks useful for default reasoning with exceptions — you can state the general rule once and add local overrides. Unlike FOL, there is no formal contradiction; specificity is the resolution principle. This approach breaks down, however, when two equally specific parents assign conflicting values.
Question 2 Multiple Choice
In a semantic network with nodes for Animal, Dog, Fido, and edges 'Dog is-a Animal,' 'Animal has-property Has-Heart,' and 'Fido is-a Dog,' how does the network establish that Fido has a heart?
AThe network cannot establish this without an explicit 'Fido has-property Has-Heart' edge
BBy traversing the is-a edges upward from Fido → Dog → Animal and collecting has-property edges encountered along the path
CBy searching the entire graph for any node labeled 'Has-Heart' and connecting Fido to it directly
DFido inherits properties only from Dog, not from Animal, so it cannot inherit Has-Heart
Inheritance is the central mechanism of semantic networks. To find Fido's properties, traverse is-a edges upward: Fido is-a Dog, Dog is-a Animal, Animal has-property Has-Heart. Following this path, Fido inherits Has-Heart without needing an explicit edge. Inheritance propagates transitively through is-a chains of any length. This is more efficient than explicitly stating every property of every individual node, and it mirrors how humans organize taxonomic knowledge.
Question 3 True / False
Semantic networks are equally expressive as first-order logic for representing knowledge — they simply use a graph notation instead of symbolic predicates and quantifiers.
TTrue
FFalse
Answer: False
Semantic networks are significantly less expressive than first-order logic. FOL can express quantified statements ('All birds can fly except those that are penguins or ostriches'), negation, logical connectives, and explicit exception handling. Semantic networks cannot represent quantifiers or logical connectives directly — they rely on inheritance conventions rather than formal rules. Additionally, multiple inheritance conflicts (two parent classes assigning different values to the same property) have no principled resolution in semantic networks, whereas FOL can represent the same situation with explicit conditions. The tradeoff is that semantic networks are more intuitive and easier to traverse for simple hierarchical queries.
Question 4 True / False
In a semantic network, a more specific node (lower in the is-a hierarchy) takes precedence over a more general ancestor when they assign conflicting property values.
TTrue
FFalse
Answer: True
This specificity convention is how semantic networks handle exceptions in default reasoning. If 'Bird can-fly' and 'Penguin cannot-fly' are both encoded, the Penguin node's more specific (directly attached) property wins. The assumption is that exceptions are stated at the most specific applicable level, and the closer (more specific) value should reflect reality better than the inherited general rule. This works well for single-inheritance hierarchies but becomes unprincipled when multiple inheritance paths assign different values — the network has no formal way to decide which parent is 'more specific.'
Question 5 Short Answer
Why does multiple inheritance create a problem for semantic networks that does not arise in the same way in first-order logic, and what is the underlying reason?
Think about your answer, then reveal below.
Model answer: In a semantic network, when a node inherits from two parent nodes that assign conflicting values to the same property, the specificity convention (closer wins) fails to resolve the conflict — both parents are equally close. The network has no principled mechanism to choose between them, creating ambiguity in any inference that depends on the disputed property. First-order logic handles this cleanly because it represents knowledge as explicit propositions with logical connectives; exceptions and conditions can be stated precisely with if-then rules and negation-as-failure or explicit exceptions. FOL can say 'X has property P unless Q holds,' whereas a semantic network has no syntax for such conditional overrides.
This limitation was a major motivation for developing more expressive knowledge representation formalisms like description logics and ontology languages (OWL), which have formal semantics and well-defined inference procedures that handle inheritance, exceptions, and disjunctions without ambiguity.