Module 4 Lexical Semantics Foundations and Issues

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Module 4 - Lexical Semantics Foundations and
Issues

• Who am I?
• Robert Belvin
• Linguistics Visiting Prof.
• USC 96
• HRL Labs (used to be Hughes Research Labs)
• Office hours just before or after class--make
  appt. via email rsbelvin_at_hrl.com
• Scope of Linguistics Component of Class
• linguistics background and areas of research
• overview areas of potential importance to NLU

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Module 4 - Linguistic Issues

• Prof. Robert S. Belvin (Linguistics Dept.)
• October 4, 6, 11, 25 November 1, 3, 29
• Topics
• Linguistic study of lexical semantics with focus
  on areas of potential benefit for automated
  systems
• Lexical semantics vs. lexicography vs. formal
  semantics
• Different views of the language faculty and how
  that can impact how we do semantic representation
  and mapping
• Some classics Fillmore (Cases),
  Gruber(Thematic Relations)
• Lexical semantic decomposition
• some semantic primitives, cross-categorial
  semantic features, languages which have
  transparent decomposition/morphologically complex
  forms, lexicalization patterns (in English and
  cross-linguistically)

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Module 4 - Linguistic Issues

• Topics (continued)
• Verb Classes and Alternations
• valence-changing operations (in English and
  cross-linguistically)
• the "unaccusative" hypothesis
• Some Formalisms and Notation
• Lexical Conceptual Structure
• Parsons Event Semantics
• Image-schemas also some Cognitive
  Semantics/metaphor - guest lect. (Dr. Tim
  Clausner).
• Mapping
• how do you get semantic representation out of
  syntactic structure?
• subcategorization and selection
• thematic role hierarchy and universal theta
  alignment hypothesis
• semantic contribution of syntactic structure

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What is Lexical Semantics?

• the semantics of lexical items (words)
• isnt semantics in linguistics always about
  words?
• lexical vs. propositional is proper contrast
• propositional John killed Bill P is all you
  get,
• or FOL representation kill (j,b) (just tells you
  2 args)
• lexical asks question how does kill mean what
  it does?

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What is Lexical Semantics?

• the semantics of lexical items (words)
• lexical semantics is often contrasted with formal
  semantics
• but this is misleading, though it is generally
  true that formal semantics, with a few
  exceptions, does not ask questions like
• how does kill mean what it does?
• in formal semantics, kill is usually taken as a
  unit
• The simple FOL meaning of Joe killed a rat
  is
• ?x (rat(x) ? kill(Joe,x))

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Lexical versus Formal

• Somewhat artificial distinction
• Many highly formalized Lexical Semantic studies
  (e.g. Pustejovsky, Wierzbicka)
• Pustejovskys representation of the template for
  an Agentive change-of-state predicate (like bake
  or kill)
• ?y?x?e1?e2?P?R ? AGENTIVER(e1,x,y)?FORMALP(
  e2,y)?e1lt?e1
• Formal semantics tradition looking within words
  (e.g. Terence Parsons)
• ?ekilling(e) ? Agent(x,e) ? Patient(y,e) ?
  Cul(e)
• (this is the meaning of kill )

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Lexical versus Formal

• Somewhat artificial distinction
• There are many mechanisms within the field of
  formal semantics that one could use for lexical
  semantic characterizations
• could do lexical properties as meaning postulates
  (entailments), etc.
• e.g. kill-event die-event
• kill-event cause-event
• etc.
• lambda-abstract templates (like Pustejovsky)
• many others

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Both formal and lexical traditions have benefits

• Formal Semantics has mathematical (set theory)
  and formal logic traditions for operating on
  representations
• Well-known logical proof methods Modus Ponens,
  Modus Tollens, etc. good for reasoners
• P ? Q and P from this infer Q
• P ? Q and Q from this infer P etc.
• DeMorgan laws
• handy for converting and to/from or
• (P ? Q) ? (P ? Q)
• (P ? Q) ? (P ? Q) etc.
• A LOT of these double negation, contraposition,
  etc.. BUT non-compliant NL exs!
• Negating a universal or existential
  quantification
• Negate the proposition AND toggle quantifier
  (between ? and ?)
• ?x P(x) ? ?x P(x) similarly with ?x ?x
  P(x) ? ?x P(x)
• But in fact these arent excluded from use in
  Lexical Semantics

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Both formal and lexical traditions have benefits

• Lexical Semantic Structures more closely resemble
  syntactic structures
• EVENTCAUSE (THING Joe , EVENT DIE (THING
  rat ))

EVENT
CAUSE
EVENT
THING
DIE
Joe
THING
rat
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Lexical vs. Formal Semantics

• Difference is really more a question of focus
• formal semantics tradition concerned with
  problems which are as much philosophical as
  semantic, such as reference and quantification
  much is based on set theory. Notion of set is
  central premise. Difficulties arise with real
  world (fuzziness, prototypes, etc.)
• lexical semantics tradition has always been
  multilingual, borrowing a lot from descriptive
  linguistics and cognitive psychology. Some
  accept set as axiomatic, others dont.

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Lexical vs. Formal Semantics

• in-class mapping exercise

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Lexical vs. Formal Semantics

• BUT In spite of usefulness of formal semantics
  for developing systems for encoding lexical
  semantic knowledge, there is potential for
  advances to be made in understanding of
  I-language which are not likely with formal
  semantics.
• different goals of two disciplines linguistics
  and computational linguistics cases where
  notation and formalism have allowed for genuine
  progress to be made
• often-recited distinction between building birds
  versus planes also applies are we building a
  brain or a question-answering machine?
• answer will affect how you proceed and priorities