MODELS OF COGNITION: THE CLASSICAL/CONNECTIONIST DEBATE Suzanne E. McCalden Submitted in partial fulfillrnent of the requirernents for the degree of Masters of Arts Dalhousie University Halifax, Nova Scotia August, 2000 @ Copyright by Suzanne E. McCalden National Library Bibliothèque nationale I*l of Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395, nie Wellingtm Ottawa ON K1A ON4 UnawaON K1AW Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence dowing the exclwive pemettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seii reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic fonnats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts firom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent êeimprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. For David, Richard, Gordon, Douglas, and especially Evelyn . TABLE OF CONTENTS Table of Contents v Acknowledgements vii Introduction 1 Chapter One: The Classical Theory of Cognition 4 Chapter Two: Comectionisrn and the Systematicity 34 Challenge Chapter Three: Connectionism v. The Language of 69 Thought Hypothesis Bibliography ABSTRACT Fodorts the language of thought hypothesis (LOT) and Smolensky's connectionism are examined. The systematicity debate is also examined. Fodor and Pylyshyn are correct in claiming that Smolensky's connectionisrn does not provide an account of systematicity. However, it is argued that Clark's connectionism does provide an account of systematicity though it is left open as to whether such an account is merely an implementation of the LOT. It is then argued that connectionism can trivially handle other cases concerning human cognition whereas the LOT cannot. Due to this, even if at one level of analysis connectionism is an irnplementation of the LOT, it offers a more viable and robust theory of huma cognition. ACKNOWLEDGEMENTS To begin with, 1 would like to thank my various undergraduate professors who sparked my interest in philosophy. In particular. 1 am indebted to Andrew Irvine, Alan Richardson, and Kate Talmage. 1 would also like to thank the members of my thesis committee: Mike Hymers. my third reader; and Duncan MacIntosh, for coming aboard my thesis committee as second reader; and for reading and commenting on earlier drafts. Most of all, 1 would especially like to thank Chris Viger. my thesis supervisor, who, through his lectures in philosophy of mind and brain, inspired me to write my thesis on this subject. In addition. 1 would like to thank Chris for reading and comenting on al1 drafts; and for his enormous help in shaping the ideas presented in my thesis. INTRODUCTION: In 1975, Jerry Fodor published the book entitled The Language of Thought Hypothesis, which offered a theory of human cognition and which subsequently became the cornerstone theory in philosophy of mind and cognitive science. However, in the 1980fs, a new alternative theory of human cognition emerged -- connectionism. As with many competing theories, a debate ensued concerning what is referred to as 'systematicity': our ability to entertain certain thoughts is intrinsically connected to our ability to entertain certain others. The nature of the debate is this. Those in the LOT camp, such as Fodor and Zenon Pylyshyn, daim that the LOT provides an account of systematicity but that connectionism does not. Those in the connectionism camp, such as Paul Smolensky and Andy Clark, daim that connectionism does provide an account of systematicity. The challenge that Fodor and Pylyshyn presented to the connectionists is this : ( 1) if mental representations exhibit constituent structure in connectionist models, then connectionism does not provide a novel account of systematicity. Simply, ccnnectionism is merely an implementation of the 2 LOT; and (2) if mental representations do not exhibit constituent structure in connectionist models, then systematicity is simply a mystery. In chapter one I provide an account of the Classical theory of human cognition, which includes the Computational Theory of Mind which claims that mental processes are simply computational processes, and the LOT. In chapter two 1 provide an account of connectionism. 1 then present the systematicity challenge and examine the central elements of the debate as presented by Fodor and Pylyshyn in the LOT camp and by Smolensky in the comectionist camp. The upshot of the debate is that Smolenskyfs argument in favour of connectionism does not provide an adequate account of systematicity. 1 then indicate that, by following Clark's argument, connectionism does offer an account of systematicity. However, it is left open as to whether connectionism is merely an implementation of the LOT. In chapter three I attempt to weaken Fodor's account of systematicity. I argue that there are certain cases concerning our ability to have certain thoughts, which are not entirely systematic. I then indicate that there are certain aspects concerning human cognition which connectionism can handle trivially but which the LOT cannot. Hence the thesis is that connectionism is a better mode1 of human cognition, in al1 its variations, even if the way it explains systematicity is by implementing a language of thought. Chapter One: The Classical Theory of Cognition 1. symbole and the Caqputatioaal meozy of Wnd '... language is a system of symbols which we know and uset (Stainton 1996, p. 1). But what are symbols? %en we talk of syrnbols, we usually take them to be things with syntax. The syntax of a symbol is its physical property (i.e.. its shape or form). However, individual syrnbols lack semantics (Le., meaning). It is only when we syntactically combine symbols that we are able to create meaningful words and sentences. For the purpose of this thesis, we are not concerned about the lexical nature of individual words (Le., how individual words get meaning) - that is for a theory of content to adüress. Rather. we are concerned with the syntactic properties of a sentence, and hence with the syntactic properties of the symbols whi-ch comprise it and certain of its semantic properties. We are also concerned with the fact that the syntactic properties of a sentence and certain of its semantic properties are related to human cognition (i.e., thinking). In other words, we are concerned with thinking (Le., mental processes) and syntax. And since thinking involves mental processes, thinking resides in the mind, 5 which in turn involves the manipulation of the syntax of symbols. This is the view that the mind is a syntactically driven machine. This view is held by, e.g., Daniel Dennett and Jewry Fodor: '... the brain ... is just a syntactic enginet (Dennett, 1987, p. 61 emphasis in original). 'There must be mental symbols because, in a nutshell, only symbols have syntax, and our best available theory of mental processes - indeed, the only available theory of mental processes that isntt known to be false - needs the picture of the mind as syntax- driven machine (Fodor, 1990, p. 23 emphasis in original). But how do we go about showing that the mind is a syntactically driven machine? One method is via a theoretical device known as a Turing Machine, which was developed by the mathematician/logician, Alan Turing. Turing developed such a machine to formalize the notion of computation. The formalization defined a class of physical mechanisms in terms of their formal properties of symbol manipulation and showed how the physical mechanisms could solve problems that nomally require human intelligence. Humans are also physical mechanisms capable of symbol manipulation. Thus, Turing's formalization of the notion of computation implied that machines could mimic the human mind (MacDonald 1995, pp. 4-5) . Turing machines work on symbolic computation: they follow a system of rules used to manipulate symbols. One such system of rules a Turing Machine may use is propositional or truth-functional logic. Propositional logic deals with arguments, which comprise a set of sentences with one or more premises and a conclusion. which is supposed to follow from the premises. An argument is said to be valid if and only if it is impossible to have al1 true premises with a false conclusion -- that is, if the truth of the premises guarantees the truth of the conclusion. To put it in other terrns. a valid argument preserves the truth of the premises in the conclusion. Thus, a valid forma1 argument is said to be truth-preserving. Moreover, an argument is said to be valid if and only if it has a valid form. To put it another way, the validity of an argument is dependent upon its sentential form. The form of an argument has nothing to do with its subject matter; thus, its content is irrelevant to its validity. Propositional logic is a formal language, which includes a vocabulary of primitive elements (i.e.. symbols) and a set of formation rules (i.e., grammar) together with a set of axioms and/or rules of inference. The symbols are taken 8 For example, 1 can make an inference of the form P&Q, therefore P within a propositional or truth-functional logistic system. 1 am able to make such an inference irrespective of what P and Q mean. Such an inference is truth-preserving (Le., the syntactic properties of the symbols carry its semantic properties). And this is because P and Q are conjoined by the operator 'andr. The computations of a Turing machine take place on a tape, which is marked into squares.