From Syntax to Semantics

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From Syntax to Semantics

• How to get from Form to Meaning in Two different
  ways

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What is meaning?

• Connection (grounding) in something outside
  itself
• Mental concept (ideas)
• Objects and events in the world (true/false)
• Some combination of the above
• Ultimately the success of the program in which
  it is embedded

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Principle of Compositionality

• The meaning of the whole is derived from the
  meaning of the parts and the manner of their
  combination
• John, kiss, Sally
• John kissed Sally.
• Sally kissed John.

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Semantics -- For our purposes

• Formal representational language that represents
  the manner of combination
• Lexicon that connects lexical items with some
  externally grounded object, the meaning of the
  parts

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Two approaches

• Logical
• Language of formal logic
• Model (set) theoretic grounding
• Interlingual
• Specially-developed InterLingual (IL)
  Representation
• Ontology to represent word meaning
• To some extent complementary

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Logical approach

• Predicate calculus and model theory PLUS
• Extra stuff to handle some of the complexities of
  natural language, such as
• (Scope) Every man loves a woman.
• (Generics) Dogs have four legs.
• (Specificity) John wants to marry a Norwegian.
• (Intension) What if all bald men are tall?
• (Roles) The temperature is ninety and rising.

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Logical approach ? calculus

• Key idea semantic construction parallels
  syntactic construction
• John john
• sleep sleep
• John is sleeping sleep(john)
• sleep ?xsleep(x)
• John is sleeping ?xsleep(x)(john)
• Lambda conversion sleep(john)

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Logical approach possible worlds

• Instead of one model many models
• Each model is a possible world one is
  designated as real
• Temporal logic
• Modal logic
• Intensional logic

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IL approach

• Developed in the context of Machine Translation
• Interested in word sense disambiguation
• The pig is in the pen.
• The ink is in the pen.
• Non-literal language metonymy/metaphor
• The White House reported today that
• The business opened its doors in 1928.
• Inferencing for translation mismatches

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IL approach

• An Ontology, a language-independent
  classification of objects, event, relations
• A Semantic Lexicon, which connects lexical items
  to nodes (concepts) in the ontology
• An analyzer that constructs IL representations
  and selects (an?) appropriate one

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IL approach Ontology

• A classification tree in which mother node
  contains all below it, and daughter nodes are
  distinct (is-a links)
• Complications expandable to a lattice, with
  non-exclusive daughter nodes
• Inheritable features and relations (now looks
  more like a dictionary)
• Instances can hang from bottom nodes (providing
  grounding)

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Semantic lexicon

• Provides a syntactic context for the appearance
  of the lexical item
• Provides a mapping for the lexical item to a node
  in the ontology
• Or more complex associations
• Also providing connections from syntactic context
  to semantic roles
• And constraints on these roles

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Deriving basic semantic dependency (a toy
example) Input John makes tools Syntactic
Analysis cat verb tense present subject
  root john cat noun-proper object   roo
t     tool cat noun number plural
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Relevant parts of the (appropriate sense of the
lexical entry for make) make-v1 syn-struc root
make cat v subj root var1 cat
n object root var2 cat
n sem-struc manufacturing-activity agent
var1 theme var2
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Relevant Extract from the Specification of the
Ontological Concept Used to Describe the
Appropriate Meaning of make manufacturing-activi
ty... agent human theme artifact
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Relevant parts of the (appropriate senses of
the) lexicon entries for John and tool
John-n1 syn-struc root john cat noun-proper
sem-struc human name john gender male
tool-n1 syn-struc root tool cat n sem-struc
tool
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The basic semantic dependency component of the
TMR forJohn makes tools is as follows manufac
turing-activity-7 agent human-3 theme set-1 e
lement tool cardinality gt 1
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try-v3 syn-struc root try cat v subj
root var1 cat n xcomp root
var2 cat v form OR infinitive
gerund sem-struc set-1 element-type refsem-1
cardinality gt1 refsem-1 sem event agent
var1 effect refsem-2 modality modality-
type epiteuctic modality-scope refsem-2 mod
ality-value lt 1 refsem-2 value var2 sem ev
ent
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Constructing an IL representation

• For each syntactic analysis
• Access all semantic mappings and contexts for
  each lexical item
• Create all possible semantic representations
• Test them for coherency of structure and content

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Why is Iraq developing weapons of mass
destruction?
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Concluding question

• Is all this really necessary?
• Do we need it to do Machine Translation, IR,
  IE, Q/A, summarization?
• Can we ground the symbols of language without a
  special representation of the meaning?

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Word sense disambiguation

• Constraint checking making sure the constraints
  imposed on context are met
• Graph traversal is-a links are inexpensive
• Other links are more expensive
• The cheapest structure is the most coherent
• Hunter-gatherer processing