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LING 180 Intro to Computer Speech and Language Processing - PowerPoint

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					LINGUIST 62n Language and Food

Background Lecture: Word Meaning
Oct 2, 2008 Dan Jurafsky

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Two topics today
The intuitive idea of a “word sense” Defining relations between “word senses” More carefully defining a word sense

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First idea: The unit of meaning is called a Sense or wordsense
One word “bank” can have multiple different meanings:
“Instead, a bank can hold the investments in a custodial account in the client’s name” “But as agriculture burgeons on the east bank, the river will shrink even more”

We say that a sense is a representation of one aspect of the meaning of a word. Thus bank here has two senses
Bank1: Bank2:
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Complex relationships between words and senses
Homonyms:
Two words that have the same form
– Sound the same (phonological form), written the same (orthographic form) or both

But have unrelated, distinct meanings Clear example:
– Bat (wooden stick-like thing) vs – Bat (flying scary mammal thing) – Or bank (financial institution) versus bank (riverside)

Can be homographs (bat, bank), homophones (below), or both:
– Homophones:  Write and right  Piece and peace

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Polysemy
1. The bank was constructed in 1875 out of local red brick. 2. I withdrew the money from the bank Are those the same sense? We might call sense 1:
“the building belonging to a financial institution”

Or consider the following example
While some banks furnish sperm only to married women, others are less restrictive Which sense of bank is this?

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Polysemy
We call polysemy the situation when a single word has multiple related meanings (bank the building, bank the financial institution, bank the biological repository) Most non-rare words have multiple meanings

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Polysemy: A systematic relationship between senses
Lots of types of polysemy are systematic
School, university, hospital Can all be used to mean the institution or the building.

We might say there is a relationship:
Building <-> Organization

Other such kinds of systematic polysemy:

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How do we know when a word has more than one sense?
Consider examples of the word “serve”:
Which flights serve breakfast? Does America West serve Philadelphia?

The “zeugma” test:
?Does United serve breakfast and San Jose?

Since this sounds weird, we say that these are two different senses of “serve”

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Other relationships between word meanings
Synonymy Antonymy Hypernomy Hyponomy Meronomy

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Synonyms
Word that have the same meaning in some or all contexts.
filbert / hazelnut couch / sofa big / large automobile / car vomit / throw up Water / H20

Two lexemes are synonyms if they can be successfully substituted for each other in all situations
If so they have the same propositional meaning

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Synonyms
But there are few (or no) examples of perfect synonymy.
Why should that be? Even if many aspects of meaning are identical Still may not preserve the acceptability based on notions of politeness, slang, register, genre, etc.

Example:
Water and H20 Big/large Brave/courageous

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Synonymy is a relation between senses rather than words
Consider the words big and large Are they synonyms?
How big is that plane? Would I be flying on a large or small plane?

How about here:
Miss Nelson, for instance, became a kind of big sister to Benjamin. ?Miss Nelson, for instance, became a kind of large sister to Benjamin.

Why?
big has a sense that means being older, or grown up large lacks this sense

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Antonyms
Senses that are opposites with respect to one feature of their meaning Otherwise, they are very similar!
dark / light short / long hot / cold up / down in / out

More formally: antonyms can
define a binary opposition or at opposite ends of a scale (long/short, fast/slow) Be reversives: rise/fall, up/down
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Hyponymy
One sense is a hyponym or subordinate of another if the first sense is more specific, denoting a subclass of the other
car is a hyponym/subordinate of vehicle dog is a hyponym/subordinate of animal mango is a hyponym/subordinate of fruit

Conversely
vehicle is a hypernym/superordinate of car animal is a hypernym/superordinate of dog fruit is a hypernym of mango superordinate vehicle fruit furniture mammal subordinate car mango chair dog
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Hypernymy more formally
Extensional:
The class denoted by the superordinate extensionally includes the class denoted by the hyponym

Entailment:
A sense A is a hyponym of sense B if being an A entails being a B

Hyponymy is usually transitive
(A hypo B and B hypo C entails A hypo C)

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WordNet
An on-line thesaurus/dictionary
http://www.cogsci.princeton.edu/cgi-bin/webwn

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Format of Wordnet Entries

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WordNet Noun Relations

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WordNet Hierarchies

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Now that we’ve talked about relations between senses
How do we define the sense of a word?

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Chicken in the American Heritage Dictionary
1a. The common domestic fowl (Gallus domesticus) or its young. 1b. Any of various similar or related birds. 1c. The flesh of the common domestic fowl.

2. Slang A coward.
3. Any of various foolhardy competitions in which the participants persist in a dangerous course of action until one loses nerve and stops.

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But can we define the meaning of these word senses?
From the American Heritage:

These definitions are kind of circular Fine for informal use. But can we do better?

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Classical theories of word meaning
What is the meaning of the word “square”? Four Necessary and sufficient conditions
1. 2. 3. 4. A closed flat figure Having four sides All sides are equal in length All interior angles are equal

Sometimes called a Checklist theory of meaning

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How about the words “hen”, “rooster”, “chick”
+gallus domesticus +male +adult

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What about the word “game”?
No necessary and sufficient conditions No common properties Even though each exemplar of games resembles each other We say that the concept “game” is defined more by family resemblance than by checklist.

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Name these items

Slide from Joel Cooper, University of Utah

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Basic Level Categories

chair chair furniture

office chair piano chair rocking

lamp

torchiere desk lamp
end table
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table coffee table Superordinate Basic

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More subordinate examples
Granny Smith Hatchback Manx cat Dessert spooon

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Basic level categories
The most inclusive level at which:
There are characteristic patterns of behavioral interaction A clear visiual image can be formed

Used for everyday reference

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Basic-Level Categories
Brown 1958, 1965, Berlin et al., 1972, 1973 Folk biology:
Unique beginner: plant, animal Life form: tree, bush, flower Generic name: pine, oak, maple, elm Specific name: Ponderosa pine, white pine Varietal name: Western Ponderosa pine

No overlap between levels Level 3 is basic
Corresponds to genus Folk biological categories correspond accurately to scientific biological categories only at the basic level

Slide from Ray Larson and Marc Davis

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Evidence Basic Level is Special
People almost exclusively use basic-level names in free-naming tasks Children learn basic-level concepts sooner than other levels Basic-level is much more common in adult discourse than names for superordinate categories Different cultures tend to use the same basic-level categories, at least for living things

Joel Cooper Slide

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Prototype effects
Rate the following from 1-7 as examples of vegetables
1: 2: 3: 4: 5: 6: 7: very good example good example fairly good example moderately good example fairly poor example bad example very bad example/not an example

Turnip, egglant, rhubarb Pea, zucchini, parsley Potato, lemon, tomato Carrot, cabbage
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Goodness of Exemplar tasks: Answer with “Yes” or “No”
        All robins are birds All fish can swim Some birds can swim A bat is a bird All birds can fly Some birds are fish An ostrich is a bird All birds are robins
We then measure the time it takes people to answer
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Results of these Goodness-ofExemplar tasks
Order of mention: prototypical member mentioned earlier Overall frequency: mentioned more frequently Order of acquisition: prototypical members acquired first by children Vocabulary learning: is better if definitions of new words rely on prototypes Speed of verification: faster for prototypes

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What about “bachelor”
An unmarried man Is the pope a bachelor? a newborn male baby? A man who has been living with the same woman for 40 years, they have three children, share finances? A single gay man? Definition of “bachelor” has to be understood with respect to some background frame, in this case the frame of the institution of marriage, of the norms of whatever society we are talking about.

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Frames
To understand the meaning of bachelor, or of words like “dessert” or “menu” or Chinese “siu ye”:
You need to have an entire background model of: How a European/American meal progresses and the role of sugar in the sequence (for dessert) What it means to “eat in a restaurant” (for menu) How a Chinese day progresses and the role of latenite meals and who cooks them.

It’s not clear how to represent this knowledge formally.
We saw some of this in the very long definitions in the Goddard text

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“Social division of labor” with meaning: fruit
Botantically: (American Heritage dictionary):
“The ripened ovary or ovaries of a seed-bearing plant, together with accessory parts, containing the seeds and occurring in a wide variety of forms.”

Legally: Nix v. Hedden, 149 U.S. 304 (1893) (is tomato a fruit or a vegetable wrt the Tariff Act of 1883, which taxed imported vegetables but not fruit):
“The court takes judicial notice of the ordinary meaning of all words in our tongue..Tomatoes are "vegetables," and not "fruit," within the meaning of the Tariff Act … …In the common language of the people, whether sellers or consumers of provisions, all these are vegetables which are grown in kitchen gardens, and which, whether eaten cooked or raw, are, like potatoes, carrots, parsnips, turnips, beets, cauliflower, cabbage, celery, and lettuce, usually served at dinner in, with, or after the soup, fish, or meats which constitute the principal part of the repast, and not, like fruits generally, as dessert.
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Summary
Word meaning is represented at the level of the word sense Relations between senses
Two words with same form, different meaning: homonyms A word with multiple related senses: polysemy Absolutely synonymy rare, near-synonymy more common Other sense relations: meronymy, hypernymy.

Classical (Aristotelian) definitions of meaning are based on necessary and sufficient conditions.
Don’t seem to work outside of mathematics

Whatever model we create for explaining word meaning, it must at least deal with these phenomena:
Basic level categories Prototypes Family resemblances Function as an element of meaning

Common versus botanical usage: who gets to decide?
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