From: AAAI Technical Report FS-96-04. Compilation copyright © 1996, AAAI (www.aaai.org). All rights reserved. The Syntax, Semantics, and Inference Mechanism of Natural Language Francis Y. Lin Computational Linguistics Unit School of English and Philosophy University of ~¥ales Cardiff Abstract depends on its syntax. Some KR formalisms also have a semantic component, which determines what a well- It is both desirable and plausible to treat natu- formed formula means and how meaning is ascribed to ral languageitself as a "knowledgerepresentation it. If natural language is a formalism for representing (KR) formalism. Every KRformalism has syntax and support certain inferences. The syntax of a knowledge, then its syntax provides specifications of KRformalism specifies the form in which knowl- well-formed formulae. In addition, reasoning must be edge must be encoded, and its inference mecha- performed directly on the basis of syntax, as in any nism depends on its syntax. If natural language artificial KRscheme. This is a point that has been ne- is a formalism for representing knowledge,then glected so far. If we apply this requirement to existing its syntax provides specifications of well-formed formulae. In addition, reasoning must be done accounts for syntax, we will easily find they are inade- directly on the basis of syntax, as in any arti- quate, that is, existing syntactic rules are not designed ficial KRscheme. In this respect, existing syn- to support inference and it is doubtful that they can tactic theories are inadequate, because syntax in be used directly to perform reasoning. What we need these theories does not support reasoning. In this then is a new conception of syntax of natural language, paper I present a new conception of the syntax, semantics, and inference mechanismof natural and a corresponding new conception of semantics and language. The central ideas are (1) that worduse reasoning. determines the forms of sentences, and (2) that these forms are used to express meaningand per- Syntax and Word Use form inference. Syntax, semantics, and inference are seen as an integrated whole, rather than sep- Ordinary speakers can potentially recognise and pro- arate and autonomousfields as in manyexisting duce an infinitely large number of sentences. This studies of natural language. phenomenon is called the competence of the language speaker (Chomsky, 1957). As with Chomsky, this phe- nomenon can only be accounted for by assuming that Introduction the language speaker utilises a finite set of grammar Existing knowledge representation (KR) systems rules which are capable of generating an infinite set of not use natural language directly or fully. The com- sentences. One major goal of generative grammar is mon practice is to translate natural language into an to account for this phenomenon. A generative gram- artificial formalism, on which subsequent operations mar typically consists of a set. of abstract rules and arc based. There have been attempts to make KR principles, from which all sentences in a language can formalisms close to natural language to gain wider lin- be derived. These rules and principles are so abstract guistic coverage or to obtain more efficient reasoning that they can hardly be said to be knownto the ordi- (Barwise L: Cooper, 1981; Kayser, 1987; Alshawi, 1992; nary speaker in any real sense. So they are assumed Iwanska, 1992). But, since people use natural language to be ’tacitly ~ knownto the speakers, or even ’innate" to exchange information, to gain new knowledge, to ex- in their genes (Chomsky, 1988). press thought, to perform reasoning, and so on, it is not Chomsky’s conception of grammar rules is in di- only desirable but also plausible to treat natural lan- rect contrast to that of Wittgenstein. According to guage itself as a KR formalism. Every KR formalism the later Wittgenstein, language consists of language has syntax and supports certain inferences. The s3/ntax games which are played according to rules (Wittgen- of a KR formalism specifies the form in which knowl- stein, 1972). Rules must be able to be expressed and edge must be encoded, and its inference mechanism they must be transparent to participants in a rule- i01 governed practice (Baker &: Hacker, 1985, pp. 62-3). (15) It’s kind of somebody to do something. In this section I outline an account of grammarwhose (16) *It’s hungry of somebodyto do something. rules are actually knownto the speakers. I shall start It is easy to see that usaxes, as those illustrated, are with the notion of sentence structure and try to expli- general w~vs of how words are used and that they are cate the notion of grammar via some examples. finite in number. Usaxes are grammarrules, according Consider the following sentence: to which we recognise sentences and determine their (1) John kissed Mary in front of Peter oll purpose, grammaticality. We have learned them, and they are because he wanted to annoy him. knownto us. In the rest of this section I shall show The structure of the sentence, in my analysis, is: how this finite number of explicit grammar rules can (2) somethingx happened because vsomething generate an infinite number of sentences. happened1 Usaxes are concrete, in the sense that they are ex- where somethingx happened refers to: pressible in natural language. On the other hand they (3)2 John kissed Mary in front of Peter on purpose. are also general. The more general a usax is the more This sentence in turn has a structure, which is: sentences it can cover, that is, it can generate. For (4) somebody did something on purpose. example, there exists an increasing order of generality where somebody is John, somebody did something is: amongthe following sentences: (5) John kissed Mary in front of Peter. (5) < (6) < (17) < (18) The structure of sentence (5) is: where (17-19) are: v.(6) solnebodyx did something in front of somebody (17) Somebody did something. where somebody~ did something refers to: (18) Something happened. (7) John kissed Mary. (19) Somethingis the case. whose structure is: (19) covers events, states, properties, and whatever you v.(8) somebody~ kissed somebody have. In fact (19) is the most general form of sentences It can be seen froln the above example that what I (or usaxes). call structures are expressions of word use. For exam- The process of generalisation starts, as illustrated pie, (2, 4, 6, and 8) show how the words (and phrases) above, from any sentence and ends in the most gen- ’because’, ’on purpose’, ’in front of’, and ’kiss’ are used; eral form, which is (19). What is the converse pro- and one only needs a little reflection to confirm that cess? It is an easy matter to see that it is generation, this is the case. I shall therefore call a structure like (2) 3 in the Chomskyan sense. For example, one can be- a usax, and say that a word is used grammatically if it gin with (19), through (18), (17), and (6), conforms to its usax (or one of its usaxes if it has more with (5). 4 Notice that each general usax can generate than one). We can now have a better idea about the a large number of usaxes which are less general: and notions of sentence structure and can appreciate the the latter are capable of generating a great manyeven equivalence relationship between the following propo- less general usaxes. Note also that each least general sitions: usax can still generate a large numberof concrete sen- (9) A string of words is a sentence. tences. Through this process a grammar can generate (10) The string of words has a sentence structure. very many sentences. To ensure that a grammar can (11) The sentence is gralnmatical. generate an infinite numberof sentences, recursion will (12) All the words have been used grammatically. have to be build into it. But I shall say no more about A few other examples may help to clarify the no- this here. tions of structure, usax, and grammaticality. (13) To sum up, usaxes are grammarrules, they are finite a grammatical sentence, it conforms to the structure and are knownto the speakers. Through the process of (15), which is a usax for ’kind’. (14) is ungralnmatical, generalisation, one gets more and more general usaxes because there is no such usax as (16): and through the converse process, which is generation, (13) It’s kind of you to come to see me. one produces (or can produce) all the sentences in (14) *It’s hungry of you to come to see me. language. 11 use somebodyxto meanthat there is an entity x which is referred to by the word ’somebody’.The referents will 4But one does not need to always start from (19) be left out whenthere are no ambiguities present. generate a sentence. One can start from anywhere in the 2For simplicity the analysis of the second half of (1), chain of generation. Note also that one does not ahvays which is ’he wanted to annoyhim’, will be omitted here. need to reach the end of the generation process either. One 3This notion is employedto stress the relationship be- can stop in the midway,to arrive at certain sentences: e.g. tween the use of words and the syntax of sentences. (17). 102 Semantics and Inference ing roles to objects mentioned in a sentence. Another Semantics is the study of meaning. My aims in this problem is that roles are usually not capable of cap- turing enough, let alone complete, meaning. Consider section are to present my view on meaning, and to provide justification for it. I shall show that the the- ’John dried the shirt’ for instance, the roles can not ory developed here subsumes several major semantic capture the meaning such as ’the shirt was wet and theories.