FALLIBLE RATIONALISM AND MACHINE TRANSLATION Geoffrey Sampson Department of Linguistics ~ Modern English Language University of Lancaster LANCASTER LAI-4YT, G.B. ABSTRACT Approaches to MT have been heavily influenced which might take the form of successive modific- by changing trends in the philosophy of language ations of an internal model of reality that could and mind. Because of the artificial hiatus which be described as "inferencing". followed the publication of the ALPAC Report, MT research in the 197Os and early 198Os has had to Chomskyan rationalism is undoubtedly more catch up with major developments that have occurred satisfactory as an account of human cognition than in linguistic and philosophical thinking; current- Skinnerian behaviourism. By the late 197Os, how- ly, MT seems to be uncritically loyal to a para- ever, the mechanical determinism that is part of digm of thought about language which is rapidly Chomsky's view of mind appeared increasingly unre- losing most of its adherents in departments of alistic to many writers. There is little empirical linguistics and philosophy. I argue, both in support, for instance, for the Chomskyan assumpt- theoretical terms and by reference to empirical ions that the child's acquisition of his first research on a particular translation problem, that language, or the adult's comprehension of a given the Popperian "fallible rationalist" view of mental utterance, are processes that reach well-defined processes which is winning acceptance as a more terminations after a given period of mental pro- sophisticated alternative to Chomskyan "determin- cessing -- language seems typically to work in a istic rationalism" should lead MT researchers to more "open-ended" fashion than that. Within redefine their goals and to adopt certain current- linguistics, as documented e.g. by Moore ~ Carling ly-neglected techniques in trying to achieve those (1982), the ChomsMyan paradi~ is hy now widely goals. rejected. The view which is winning widespread accept- I. Since the Second World War, three rival views ance as preserving the merits of rationalism of the nature of the human mind have competed for while avoiding its inadequacies is Karl Pepper's the allegiance of philosophically-minded people. falllbilist version of the doctrine. On this Each of these views has implications for our account, the mind responds to experiential inputs understanding of language. not by a deterministic algorithm that reaches a halt state, but by creatively formulating fallible The 195Os and early 1960s were dominated by s conjectures which experience is used to test. behaviourist approach tracing its ancestry to John Typically the conjectures formulated are radically Locke and represented recently e.g. by Leonard novel, in the sense that they could not be pre- Bloomfield and B.F. Skinner. On this view, "mind" dicted even on the basis of ideally complete is merely a name for a set of associations that knowledge of the person's prior state. This have been established during a person's life version of rationalism is incompatible with the between external stimuli and behavioural responses. materialist doctrine that the mind is nothing but The meaning of a sentence is to be understood not an arrangement of matter and wholly governed by as the effect it has on an unobservable internal the laws of physics; but, historically, material- model of reality but as the behaviour it evokes in ism has not commonly been regarded as an axiom the hearer. requiring no argument to support it (although it may be that the ethos of Artificial Intelligence During the 1960s this view lost ground to the makes practitioners of this discipline more than rationalist ideas of Noam Chomsky, working in an averagely favourable towards materialism). intellectual tradition founded by Plato and rein- augurated in modern times by Hone Descartes. On As a matter of logic, fallible conjectures in this view, stimuli and responses are linked only any domain can be eliminated by adverse experience indirectly, via an immensely complex cognitive but can never be decisively confirmed. Our mechanism having J ts own fixed principles of oper- reaction to linguistic experience, consequently~ ation which are independent of experience. A is for a Popperian both non-deterministic and given behaviour is a response to an internal mental open-ended. There is no reason to expect a person event which is determined as the resultant of the at any age to cease to improve his knowledge of initial state of the mental apparatus together with his mother-tongue, or to expect different members the entire history of inputs to it. The meaning of of a speech-community to formulate identical a sentence must be explained in terms of the unseen internalized grammars; and understanding an indiv- responses it evokes in the cognitive apparatus, idual utterance is a process which a person can 86 execute to any desired degree of thoroughness -- 3. What are the implications for MT, and for AI we stop trying to improve our understanding of a in general, of a shift from a deterministic to a particular sample of language not because we reach fallibilist version of rationalism? (On the general issue see e.g. the exchange between a natural stopping-place but because we judge that Aravind Joshi and me in Smith 1982.) They can be the returns from further effort are likely to be summed Up as follows. less than the resources invested. First, there is no such thing as an ideal For a Chomskyan linguist, divergences between speaker's competence which, if simulated mechanic- individuals in their linguistic behaviour are to be ally, would constitute perfect MT. In the case of explained either in terms of mixture of "dialects" "literary" texts it is generally recognised that or in terms of failure of practical "performance" different human translators may produce markedly fully to match the abstract "competence" possessed different translations none of which can be con- by the mature speaker. For the Popperian such sidered more "correct" than the others; from the divergences require no explanation; we do not Popperian viewpoint literary texts do not differ possess algorithms which would lead to correct qualitatively from other genres. (Referring to results if they were executed thoroughly. Indeed, the translation requirements of the Secretariat of since languages have no reality independent of the Council of the European Communities, P.J. their speakers, the idea that there exists a Arthern (1979: 81) has said that "the only quality "correct" solution to the problem of acquiring a we can accept is i00~0 fidelity to the meaning of language or of understanding an individual sent- the original". From the fallibilist point of view ence ceases to apply except as an untheoretical that is like saying "the only kind of motors we approximation. The superiority of the Popperian are willing to use are perpetual-motion machines".) to the Chomskyan paradigm as a framework for interpreting the facts of linguistic behaviour is Second, there is no possibility of designing argued e.g. in my Making Sense (1980), Popperian an artificial system which simulates the actions Linguistics (in press). of an unpredictably creative mind, since any machine is a material object governed by physical law. Thus it will not, for instance, be possible 2. There is a major difference in style between to design an artificial system which regularly the MT of the 1950s and 1960s, and the projects of uses inferencing to resolve the meaning of given the last decade. This reflects the difference texts in the same way as a human reader of the between behaviourist and deterministic-rationalist texts. There is no principled barrier, of course, paradigms. Speaking very broadly, early MT to an artificial system which applies logical research envisaged the problem of translation as transformations to derive conclusions from ~iven that of establishing equivalences between observ- premisses. But an artificial system must be able, surface features of languages: vocabulary restricted to some fixed, perhaps very large, data- items, taxemes of order, and the like. Recent MT base of premisses ("world knowledge"). It is research has taken it as axiomatic that successful central to the Popperian view of mind that human MT must incorporate a large AI component. Human inferencing is not limited to a fixed set of pre- translation, it is now realized, involves the misses but involves the frequent invention of new understanding of source texts rather than mere hypotheses which are not related in any logical transliteration from one set of linguistic con- way to the previous contents of mind. An MT ventions to another: we make heavy use of infer- system cannot aspire to perfect human performance. encing in order to resolve textual ambiguities. (But then, neither can a human.) MT systems must therefore simulate these inferenc- ing processes in order to produce human-like out- Third: a situation in which the behaviour of put. Furthermore, the Chomskyan paradigm incorp- any individual is only approximately similar to orates axioms about the kinds of operation char- that of other individuals and is not in detail acteristic of human linguistic processing, and MT predictable even in principle is just the kind of research inherits these. In particular, Chomsky situation in which probabilistic techniques are and his followers have been hostile to the idea valuable, irrespective of whether or not the pro- that any interesting linguistic rules or processes cesses occurring within individual humans are might be probabilistic or statistical in rmture themselves intrinsically probabilistic. To draw (e.g. Chomsky 1957: 15-17, and of. the controversy an analogy: life-insurance companies do not con- about Labovian "variable rules"). The assumption demn the actuarial profession as a bunch of cop- that human language-processing is invariably an outs because they do not attempt to predict the all-or-none phenomenon might well be questioned precise date of death of individual policyholders.