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Published on November 26, 2007

Author: avsar

Source: authorstream.com

Predicate Calculus:  Predicate Calculus Representing meaning Revision:  Revision First-order predicate calculus Typical “semantic” representation Quite distant from syntax But still clearly a linguistic level of representation (it uses words, sort of) Types of representation:  Types of representation The man owned the gun which he used to shoot an elephant 5. Predicate calculus An elephant was shot by the man with his gun The man used his gun to shoot an elephant event(e) & time(e,past) & pred(e,shoot) & man(a) & the(a) & (b) & dog(b) & shoot(a,b) & (c) & gun(c) & own(a,c) & use(a,c,e) The man shot an elephant with his gun The man used the gun which he owned to shoot an elephant First-order predicate calculus:  First-order predicate calculus Computationally tractable Well understood, mathematically sound Therefore useful for inferencing, expressing equivalence Can be made quite shallow (almost like a deep structure), or quite abstract Good for expressing facts and relations Therefore good for question-answering, information retrieval First-order predicate calculus:  First-order predicate calculus Predicates – express relationships between objects, e.g. father(x,y), or properties of objects, e.g. man(x) Functions –can be evaluated to objects, e.g. fatherof(x) Constants – specific objects in the “world” being described Operators (and, or, implies, not) and quantifiers (, ) Logic operators and quantifiers:  Logic operators and quantifiers Universal quantifier  (‘all’) All dogs are mammals: x dog(x)  mammal(x) Dogs are mammals, The dog is a mammal A dog is a mammal Existential quantifier  (‘there exists’) John has a car : x car(x) & own(john,x) Quantifier scope:  Quantifier scope Every man loves a woman Ambiguous in natural language x man(x) x woman(y) love(x,y) x woman(y) x man(x) love(x,y) Every farmer who owns a donkey beats it What does ‘it’ refer to? x (farmer(x) & y donkey(y) own(x,y))  beat(x,y) Quantifiers:  Quantifiers Natural language has many and various quantifiers, some of which are difficult to express in FOPC: many, most, some, few, one, three, at least one, ... often, usually, might, ... Ambiguity with negatives:  Ambiguity with negatives Every student did not pass an exam x student(x) x exam(y) pass(x,y) y exam(y) x student(x) pass(x,y) x student(x) x exam(y) pass(x,y) All women don’t love fur coats No smoking seats are available I don’t think he will come (neg raising) I don’t know he will come ~ I know he won’t come Combinatorial explosion:  Combinatorial explosion Quantifier ambiguities can be compounded “Many people feel that most sentences exhibit too few quantifier scope ambiguities for much effort to be devoted to this problem, but a casual inspection of several sentences from any text should convince almost everyone otherwise.” (Jerry Hobbs) On top of other ambiguities (e.g. attachment) First-order predicate calculus:  First-order predicate calculus In a quite shallow FOPC representation we can closely map verbs, nouns and adjectives onto predicates man(x), fat(x), standup(x), see(x,y), give(x,y,z) Proper names map onto objects, e.g. man(john), see(john,mary) Slide12:  Grammatical meanings can be expressed as predicates e.g. A man eats icecream with a spoon X man(x) & y icecream(y) & z spoon(z) & eats(x,y) & uses(x,z) A man shot an elephant in his pyjamas x man(x) & y elephant(y) & shot(x,y) & z pyjamas(z) & owns(x,z) & ... wearing(x,z) loc(y,z) (wearing(x,z) | wearing(y,z) | loc(y,z)) | loc(x,z)) wearing(y,z) loc(x,z) First-order predicate calculus:  First-order predicate calculus We can use operators of predicate calculus to express aspects of meaning that are implicit, and thereby extract new meaning from new utterances e.g. eats(x,_) & uses(x,y)  holds(x,y) Or make inferences e.g. gives(x,y,z)  has(x,z) &  has(x,y) Tense and time:  Tense and time Representing text, we need to represent tense John eats a cake X cake(X) & eats(john,X) John ate a cake X cake(X) & ate(john,X) X cake(X) & eats(john,X,past) X cake(X) & eats(john,X,pres) event(E) eating(E) & agent(E,john) & X cake(X) & object(E,X) & past(E) time(E,past) Tense and time:  Tense and time Relationship between tense and time by no means straightforward I fly to Delhi on Monday I fly to Delhi on Mondays I fly to Delhi and find they have lost my luggage I fly to Delhi if I win the competition He will be in Delhi now You might want a deeper representation rather than just a mirror of the surface tense Tense and time:  Tense and time Reichenbach’s approach Tense is determined by three perspectives: Event time Reference time Utterance time These can be ordered relative to time Also, they can be points or durations Tense and time:  Tense and time I had eaten E < R < U I ate E=R < U I have eaten E < R=U I eat E=R=U I will eat U=R < E I will have eaten U < E < R Linguistic issues:  Linguistic issues There are many other similarly tricky linguistic phenomena Modality (could, should, would, must, may) Aspect (completed, ongoing, resulting) Determination (the, a, some, all, none) Fuzzy sets (often, some, many, usually) Semantic analysis:  Semantic analysis Syntax-driven semantic analysis Compositionality Semantic grammars Procedural view of semantics Syntax-driven semantic analysis:  Syntax-driven semantic analysis Based on syntactic grammars CFG rules augmented by semantic annotations Compositionality Meaning of the whole is the sum of the meaning of its parts But not just the parts, but also the way they fit together Pipeline architecture:  Pipeline architecture Semantic augmentations to PSG rules - example:  Semantic augmentations to PSG rules - example NP  det, adj, n {sem(NP,X) = qtf(det,X) sem(adj,X) & sem(n,X)} a = det {qtf(X,exists(X))} fat = adj {sem(X,fat(X))} man = n {sem(X,male(X) & hum(X)} a fat man exists(X) fat(X) & male(X) & hum(X) Semantic augmentations to PSG rules - example:  Semantic augmentations to PSG rules - example S  NP, VP {sem(S,X,Y) = sem(NP,X) & sem(VP,X,Y)} NP  det, adj, n {sem(NP,X) = qtf(det,X) sem(adj,X) & sem(n,X)} VP  v, NP {sem(VP,X,Y) = sem(v,X,Y) sem(NP,Y)} eats = v {sem(X,Y,eats(X,Y) & tense(pres)} cake = n {sem(X,cake(X)} a fat man eats a cake exists(X) fat(X) & male(X) & hum(X) & exists(Y) & cake(Y) & eats(X,Y) & tense(pres) How to do this:  How to do this Quite complex Fortunately, there is a mechanism Lambda calculus (Church 1940) See J&M ;-) Such representations often called “quasi logical forms” because of their (too) close relation to syntax

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