The preceding chapter presented the basic difficulties associated with producing semantic representations of sentences in context. This chapter surveys several well-known natural language processors, concentrating on their efforts at overcoming these particular difficulties. The processors use different styles of semantic representation as well as different methods for producing the chosen semantic representation from the syntactic parse. Ideally, clearly defined methods of producing semantic representations should be based on a linguistic theory of semantic analysis; a theory about the relationships between the given syntactic and semantic representations, and not just on the particular style of semantic representation. Computational linguistics has a unique contribution to make to the study of linguistics, in that it offers the opportunity of realizing the processes that must underlie the theories. Unfortunately, it seems to be the case that those systems that adhere most closely to a particular linguistic theory have the least clearly defined processing methods, and vice versa.

Another important aspect to examine is whether or not any of the methods make significant use of procedural representations. An important contribution hoped for from computational linguistics is an understanding of procedural semantics as “a paradigm or a framework for developing and expressing theories of meaning” [Woods, 1981, p. 302]. It is argued that adding procedures to a framework should greatly enrich its expressive power [Wilks, 1982]. In spite of the intuitive appeal of this argument, much work remains to be done before the benefits can be convincingly demonstrated.

Introduction

A primary problem in the area of natural language processing is the problem of semantic analysis. This involves both formalizing the general and domain-dependent semantic information relevant to the task involved, and developing a uniform method for access to that information. Natural language interfaces are generally also required to have access to the syntactic analysis of a sentence as well as knowledge of the prior discourse to produce a detailed semantic representation adequate for the task.

Previous approaches to semantic analysis, specifically those which can be described as using templates, use several levels of representation to go from the syntactic parse level to the desired semantic representation. The different levels are largely motivated by the need to preserve context-sensitive constraints on the mappings of syntactic constituents to verb arguments. An alternative to the template approach, inference-driven mapping, is presented here, which goes directly from the syntactic parse to a detailed semantic representation without requiring the same intermediate levels of representation. This is accomplished by defining a grammar for the set of mappings represented by the templates. The grammar rules can be applied to generate, for a given syntactic parse, just that set of mappings that corresponds to the template for the parse. This avoids the necessity of having to represent all possible templates explicitly. The context-sensitive constraints on mappings to verb arguments that templates preserved are now preserved by filters on the application of the grammar rules.

This chapter presents the semantic processor that performs the semantic role assignments at the same time as it is decomposing the verb representation. Chapter 3 has described how semantic roles are defined as arguments to the semantic predicates that appear in the lexical entries. These arguments are instantiated as the lexical entries are interpreted. A possible instantiation of a predicate-argument is the referent of a syntactic constituent of the appropriate syntactic and semantic type. The syntactic constituent instantiations correspond to the desired mappings of syntactic constituents onto semantic roles. Other instantiations can be made using pragmatic information to deduce appropriate fillers from previous knowledge about other syntactic constituents or from general world knowledge.

These tasks are performed by interpreting the lexical entries procedurally similarly to the way that Prolog interprets Horn clauses procedurally [Kowalski, 1979]. The lexical entries are in fact Horn clauses, and the predicate-arguments that correspond to the semantic roles are terms that consist of function symbols with one argument. The procedural interpretation drives the application of the lexical entries, and allows the function symbols to be “evaluated” as a means of instantiating the arguments. The predicate environments associated with the mapping constraints correspond to states that may or may not occur during the procedural interpretation of the entries. Thus the same argument can be constrained differently depending on the state the verb interpretation is in. The state can vary according to instantiations of arguments or by the predicates included in the predicate decomposition.

1. 1. (Humphrey, p. 84, no. 2)

2. Two pulleys of weights 12 lb and 8 lb are connected by a fine string hanging over a smooth fixed pulley. Over the former is hung a fine string with weights 3 lb and 6 lb at its ends, and over the latter a fine string with weights 4 lb and x lb. Find x so that the string over the fixed pulley remains stationary, and find the tension in it.

3. 2. (Part of Humphrey, p. 75, nos. 566)

4. A mass of 9 lb resting on a smooth horizontal table is connected by a light string, passing over a smooth pulley at the edge of the table to a mass of 7 lb hanging freely. Find the common acceleration, the tension in the string and the pressure on the pulley.

5. 3. Two particles of mass B and C are connected by a light string passing over a smooth pulley. Find their common acceleration.

6. 4. Particles of mass 3 and 6 lb are connected by a light string passing over a smooth weightless pulley; this pulley is suspended from a smooth weightless pulley and offset by a particle of mass 8 lb. Find the acceleration of each particle.

7. 5. A man of 12 stone and a weight of 10 stone are connected by a light rope passing over a pulley. Find the acceleration of the man. If the man pulls himself up the rope so that his acceleration is one half its former value, what is the upward acceleration of the weight?

8. […]

This chapter presents the formalization of the pulley domain. In this domain, the entities involved tend to be simple solid entities like particles and strings, while the relationships between them include notions of support, contact, or motion of some form. Section 3.2 describes the formalization of the pulley world in terms of the types of entities and their properties. The relationships are used for the decompositions of the verbs which are described in section 3.3 where the lexical entries of the verbs are listed. Each verb is subcategorized in terms of the primary relationship involved in the decomposition. The semantic roles are arguments of these relationships. The lexical entries include the decompositions of these primary relationships. Section 3.5 introduces the mapping constraints for assigning syntactic constituents to semantic roles. Examples demonstrate how the syntactic cues can be used with predicate environments to preserve the same semantic role interdependences that are preserved by templates. The last section describes the semantic constraints used in conjunction with the mapping constraints to test that the referent of a syntactic constituent is of the correct semantic type. The last category of constraints described, the pragmatic constraints, are used by inference-driven mapping to fill semantic roles that do not have mappings to syntactic constituents. Chapter 4 describes how inference-driven mapping interprets the lexical entries procedurally to drive the semantic analysis of paragraphs of text.

This chapter summarizes the results that have been presented in the preceding chapters, in particular the process by which inference-driven mapping goes directly from the syntactic parse of a sentence to a “deep” semantic representation that corresponds to a traditional linguistic decomposition. The summary illustrates two of the most important benefits offered by inference-driven mapping over the template approach, namely, (1) the clear distinction between the verb definition and the final semantic representation achieved, and (2) an integrated approach to semantic analysis. The first benefit is of special relevance to linguistic theories about semantic representations, in that it provides a testing ground for such theories. The second benefit is of more relevance to computational models of natural language processors, in terms of interfacing semantic processing with syntactic parsing. The last section suggests directions of future research for pursuing these objectives.

Integrated semantic analysis

As discussed in chapter 2, traditional approaches to semantic processing need several levels of description to produce a “deep” semantic representation from a syntactic parse. The most popular of these approaches, termed the template approach, can be seen as using at least two intermediate levels of description, (1) the template level which is used for assigning mappings from syntactic constituents and semantic roles, and (2) the canonical level where the semantic roles are grouped together to simplify derivation of a “deep” semantic representation. These separate levels of description impose several stages of processing on the implementations, since only certain pieces of information are available at any one stage.

Achebe was born in Eastern Nigeria on 16 November 1930. He was christened Albert Chinualumogu. His father, Isaiah Okafor Achebe, a catechist for the Church Missionary Society, had as a young man been converted to Christianity and had been an evangelist and church teacher in other parts of Eastern Nigeria before returning to settle in his ancestral village, Ogidi, five years after the birth of his fifth child. His mother, Janet, had also been a convert to Christianity. When Isaiah and Janet Achebe were married in 1909, the service was conducted by Isaiah Achebe's teacher, supervisor and friend, the eminent missionary and amateur anthropologist, G. T. Basden, who was to be the model for Mr. Brown, the more tolerant and tolerable missionary in Things Fall Apart.

Achebe describes growing up in Ogidi in the thirties as living ‘at the crossroads of cultures’:

But the ‘supposed’ parting of the ways between heathen and Christian was not absolute. As a small child, Achebe moved between both sides of his family, finding himself intrigued by the differing rituals and tempted by the ‘heathen’ food.

Joyce Cary'sMister Johnson tells the story of a young Nigerian clerk who takes a bribe, loses his job, regains it, helps and encourages the young British colonial officer Rudbeck build a road, embezzles taxes to do so, murders a white storekeeper, and is subsequently tried and executed by Rudbeck. The novel seeks above all to celebrate the character of Johnson, who is portrayed as a Dionysian character, bursting with emotion, song, dance and spontaneity, and enamoured of European civilization. He is contrasted both with the self-interested and unimaginative pagan and Muslim Africans and with the British colonial officers, who are characteristically tight-lipped, constrained by reason and the letter of the law. Johnson provides Rudbeck with the inspiration and energy as well as the labour force and the money to build a road through his district. Cary's implication is that Johnson's qualities must be linked to and put to the service of Rudbeck's vision and technological knowledge, and that Rudbeck can become more humane and more creative by learning from Johnson, although Johnson's anarchic and destructive tendencies must be suppressed. At the end of the novel, Rudbeck follows the letter of the law in trying and condemning Johnson, but bends the law by agreeing to personally execute Johnson, who pleads with him to do so, reminding Rudbeck that he (Johnson) has always regarded him as ‘his father and mother’.

Arrow of God had been concerned with the nature of Igbo religious belief and believers, portraying not fetishes and ju-ju, ‘so crude and stupid’, as Louis Aladai describes Rimi civilization in The African Witch, but a metaphysic, constructed and questioned by Igbo intellectuals and leaders, a system of beliefs fashioned to respond to and balance the demands of individual and communal well-being and to acknowledge the limits of human knowledge and power. With A Man of the People, Achebe takes up some of the other themes significant in Cary's novels, and especially The African Witch: these are the themes of interlinked political and sexual jealousy, of the young westernized idealist and would-be leader of his people, of political demagoguery, and of political abuse in a situation where African and European forms of political leadership and participation each have a particular kind of appeal and function.

Odili, the narrator and chief protagonist of A Man of the People, is like Louis Aladai a nationalist who has nothing but contempt for the traditions of his nation. He does not ‘care too much for our women's dancing’, he speaks as any foreigner might of the members of the hunters' guild as ‘these people’, ‘bush’ is the nastiest epithet he can think of, and he scornfully dismisses the people with whom he works and whose children he teaches:

Here were silly ignorant villagers dancing themselves lame and waiting to blow off their gunpowder in honour of one of those who had started the country off down the slopes of inflation.

Chinua Achebe is Africa's most widely read novelist and the first to be taken seriously by both African and European readers. His novels and critical pronouncements have profoundly influenced his readers' understanding of Africans and their lives and have formed the basis for many a discussion of ‘the African novel’. They have also provided a model for succeeding African novelists to follow and contend with. Yet, although Achebe's first and most influential novel, Things Fall Apart, was published thirty years ago, critical discussion of his work as a whole has rarely moved beyond books designed as introductions. Three of those books were first written more than fifteen years ago – although they have been revised since to take into account Achebe's short fiction and poetry – and they are concerned chiefly with describing the novels in terms of their central themes, conflicts and characters. The fourth book, Robert M. Wren's Achebe's World, was published in 1980 and provides important historical and anthropological background and annotation for the novels. These four critical works have served and continue to serve a useful purpose for new students of Achebe's writing. The present study seeks to build upon the foundation they have constructed.

That foundation contains some now solidly entrenched concepts and assumptions about the nature of Achebe's achievement, a foundation to which Chinua Achebe's own essays and lectures – notably ‘The Role of the Writer in a New Nation’ and ‘The African Writer and the English Language’ – have contributed much of the framework.