nlp_intro

13. What is knowledge about language?

Explanations and visualisations

• Crash Course Linguistics #1, #14
• L. Rizzi, Collège de France opening lecture, Complexity of linguistic structures, simplicity of language mechanisms,
• S. Piantadosi, Zipf’s word frequency law in natural language: A critical review and future directions

 

 

Most important theoretical problems

• Arbitrariness of the sign: there is no natural connection between a word and its meaning,
  • but the Bouba/kiki effect shows some connection
  • what is the mapping between the meaning and the form?
• Double articulation (duality of patterning): merging meaningless units into meaningful ones, merging meaningful units into higher-order meaningful units
  • sometimes only the latter regarded as language
  • relevant to the question of what are the smallest units of language
  • relevant to the question of the form of language competence: is it just one operation (merge)?
• Displacement: we can talk about things we don’t see
  • but it seems that we don’t use this freedom all the time
  • relevant to the question what knowledge can be extracted from texts
• Innateness: are we born with a specialised language faculty or it’s all just general cognition?
  • a famous puzzle: Poverty of the stimulus
  • distantly relevant to generalisation and universality of NLP models
  • related to universality

 

Theoretical syntax: how do we compose phrases?

Source: Growing trees: The acquisition of the left periphery

• Lexicon (vocabulary) contains two kinds of units:
  • Content units (nouns, adjective, verbs) describe entities we can imagine, e.g. things, people, animals, events and their properties
  • Function units (articles, determiners, auxiliaries, complementisers, tense, aspect, mood, conjunction, implication, negation) express relations between content units
• Elements of lexicon are terminal nodes in a syntactic tree, non-terminal nodes are categories
• Basic syntactic operations: merge, move, search, externalisation
• Grammatical notions are composed of basic operations (e.g. the notion of a relative clause)
• Merge is binary, recursive, it is not symmetric (the result of a merge is a non-terminal category of the type of one of the two merged elements), the order of merges is constant
• Animals might be able to merge, but not recursively
• Method: introspection, do we accept or not a string? E.g.:

     On croit [ que Jean est parti à 17h ]             
     'We think that John left at 17h'

     Quand croit-on [ que Jean est parti <...> ] ?  
     'When do we think that John left?'      

     On se demande [ qui est parti à 17h ] 
     'We wonder who left at 17h'

     * Quand se demande-t-on [qui est parti <...> ] ? 
     * 'When do we wonder who left ?'    

 

Information theory: text measures and quantitative laws

• Observations of strings, typically not relating to the theoretical questions
• Shannon entropy and complexity: text as a sequence of outcomes of a stochastic process
• Zipf-Mandelbrot Law: the relation between word frequency and its frequency rank is universal
• Zipf’s Law of Abbreviation: the length of a word is inversely proportional to its frequency
• Menzerath-Altmann’s Law: the bigger the whole, the smaller the parts
• Uniform information density, constant information rate: tendency to keep the amount of information constant

 

Grammar vs. linguistics

• Grammar is what we study in school: rules of a single language
• Linguistics is about explaining why grammars are the way they are
  • What is common to all grammars?
  • What kinds of rules may or may not exist?
• Most of what is called “linguistics” is about grammars (not theoretical)

 

Linguistics vs. NLP

• Symbolic rule-based methods relied a lot on grammars
• Statistical methods used annotated texts, Penn Treebank was an example for many others, now Universal Dependencies
• Both rules and annotations are slightly formalised grammars, not scientific theory
• LLMs and self-supervised learning often work without any explicit linguistic knowledge
• Popular question: is there still any room for linguistics in NLP?

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