abductive reasoning_dougla walton

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of argument diagramming, and though the square of opposition is mentioned, it isbarely explicated. Fallacies are covered, but quickly, and not as a central tool forcritical reasoning.

These comments should not be taken as indicating shortcomings. Rather, thetext is not intended for courses which now use texts such as Johnson and BlairLogical Self-Defense), Govier A Practical Study o f Argument), or Groarke and

Tindale Good Reasoning Matters). The material on which they rely, and there area great number of references, demonstrate that the fields are different. I would havepreferred if more of the work done in Argumentation Theory had been included, butperhaps limitations had to be set. In any case, as a secondary or ancillary text,Cooperative Argumentation would serve as an interesting and nicely thought out

resource for a course in critical reasoning.

eference

Gilbert, Michael A. 1995. Arguments and Arguers . Teaching Philosophy. 18: 2:125-138.

MUCHAEL A. GILBERT York University

ABDUCTIVE REASONING. By DOUGLAS N. WALTON. Tuscaloosa, AL: TheUniversity of Alabama Press, 2004. Pp. xvi, I -304. ISBN -08 173 144 I 5 US$40cloth.

Once upon a time there was a philosopher with a bag of beans. The philosopherwas Charles Sanders Peirce, and he used the beans to cultivate a seminal distinctionbetween three types of reasoning: deduction, induction, and abduction. The story

goes as follows. Suppose we know as a fact that all the beans in a bag are white andthat a handful of beans have been taken from such bag: then we can safely infer thatall these beans are white-and we do so by deduction. Now imagine instead wehave seen a number of beans being drawn from the bag, and all of them were white:even if we are not sure that every bean in that bag is white, we can reasonably infersuch general rule from the statistical correlation we observed, reasoning byinduction. Finally, let us say that again we know that all the beans in the bag arewhite, and then we notice a handful of white beans on a table nearby: under thesecircumstances, it seems reasonable to presume that those beans were drawn fromthat particular bag-and this is abduction.

More than a century later, the notion of abduction is still controversial inphilosophy, linguistics, law psychology, and computer science. Walton's AbductiveReasoning is one of the most recent attempts to struggle with this fascinatingproblem, and one of the most remarkable in its interdisciplinary breadth and scholarlyerudition. Walton conceives abduction as inference to the best explanation, and inthis book he endeavours to provide a detailed account of what exactly is meant bybest explanation . His main contribution is to outline a dialogical model o f

explanation, and apply it to the analysis of abductive reasoning. This also revealsthe connections between abduction and argumentation, since Walton's model of


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explanation is a direct extension of his dialogical account of argument structures.Moreover, his theoretical analysis is constantly informed by and tested againstseveral applicable domains, such as scientific discovery, historical research, legalreasoning, medical diagnosis, and Artificial Intelligence. As a result, Walton's bookprovides a stimulating, updated, and comprehensive survey of this interdisciplinaryfield of research, with impressive mastery of the huge bibliography on abductivereasoning.

As for the specific contents of the book, chapter I introduces the basic notionof abduction and its alleged differences from both deduction and induction, withprecise discussion of the existing literature. The problem of defining the form ofabductive inference is outlined: is abduction to be modelled as the fallacy of affirmingthe consequent (since A implies and B is the case, then A , or is it to be seen as

inference to the best explanation, where a set of data D is observed and there is ahypothesis H that would explain D better than any other available alternatives, sothat it is presumable that H is true? Walton frames this discussion using notionsand examples taken from epistemology, argumentation theories, philosophy ofscience, legal reasoning, and artificial intelligence.

Chapter 2 delineates Walton's dialogue model of explanation. This section istruly pivotal: Walton aims to analyze abduction as inference to the best explanation,so he needs to define precisely (i) what is an explanation, and (ii) what makes an

explanation more or less successful. The basic rationale o f his account is to seeexplanation as a dialogue between two agents, in which one agent is presumedby the other to understand something, and the other agent asks a question meantto enable him to understand it as well. n explanation is successful i f i t communicatesunderstanding of a sort needed to enable the questioner to make sense of the thingquestioned (p. 51). Walton's account relies on a chain of embedded concepts: adefinition o f understanding is needed to characterize explanation, which in turnserves to analyze abduction as inference to the best explanation. Walton drawsinspiration from several different domains, such as models of explanation in ArtificialIntelligence (pp. 52-60), the debate on mind-reading capabilities in humans (pp. 60-66), and pragma-dialectical taxonomies of different speech acts and conversationalcontexts (pp. 66-78). Thus he is able to put forward his own model of what constitutesan explanation, by setting the conditions (dialogical, epistemological, and pragmatic)that characterize it as a special kind of speech act (pp. 78-82).

Chapter 3 assesses the import of Walton's dialogue model of explanation for thedebate on the nature o f rationality with special reference to the recent surge o finterest for argumentation theories in artificial intelligence (computational dialectics).Most noticeably, Walton provides a clear distinction between the fun tion o fargument (to remove doubts on unsettled issues) and the function of explanation(to provide understanding of known facts), and a thorough discussion of inference-chaining in reasoning both forward (e.g., in assessing a legal case) and backward(e.g., in scientific inquiry). As for the nature of rational thought, Walton concludesthat, notwithstanding shortcomings o f both deductive and inductive models o freason, we should not give up the notion of rationality, but rather devise newmodels for it, by first recognizing that a large part of human reasoning is abductive.

One o f these new models is presented in chapter 4: defeasible modus ponens(OMP). Walton observes that several argumentation schemes (i) have a modusponens (MP) structure, albeit (ii) they are defeasible, i.e., their conclusion may have


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driven model of abduction for applications in artificial intelligence, especiallyquestion-answering technologies and critiquing systems (pp. 269-272).

On the whole, Walton's analysis of abductive reasoning is both detailed and

self-contained, with clear explanations of all key concepts and several examples.This makes the text fully accessible to non-experienced readers, while people alreadywell-acquainted with Walton's approach may find it convenient to skip those partswhich borrow more heavily from his previous works (1996; 1998). Not surprisingly,Walton's far-reaching interdisciplinary approach also has its drawbacks, alongsideits virtues. Sometimes, his perspective on abduction is so generously broad and allencompassing that a few loose ends are left to dangle behind, and some minorpoints appear either vague (e.g., characterization of reasoning on pp. 102-104),slightly superficial (e.g., discussion of mind-reading and theory of mind on pp. 60-66), or simply repetitive (e.g., readers will lose count of how many times it is saidthat abductive reasoning is defeasible, nonmonotonic, and presumptive). But theseminor shortcomings are indeed to be expected, whenever an author attempts sucha far-sighted synthesis.

More relevant is the fact that there are some (critical) questions that are raised,rather than solved, by Walton's approach to abduction. This is not necessarily adrawback, and it even helps in making the book so thought-provoking and enticing,but still it is worth considering. To my mind, the most important issues that Walton

still fails to settle are the following:1. If abduction is to be taken as inference to the best explanation, thenwhat is an explanation, and what is it that makes it 'best'?

2. What is the real import of a dialogue model of abduction?t is almost tautological to say that defining explanation is crucial for a theory of

abduction as inference to the best explanation. Walton conceives explanation as adialogue in which some understanding is transferred from one party to the other.Hence the corner-stone of abduction is the notion of understanding. In trying to

define it Walton borrows from the long debate on mind-reading i.e., the ability ofinterpreting, predicting and influencing the behaviour of other agents by usinginsight on their inner mental states. Walton suggests that understanding the actionsof another agent requires what he calls simulative practical reasoning (p. 65): thecapacity of (i) recognizing as familiar the predicament in which we observe the otheragent, and hence (ii) drawing the relevant conclusion on what the other is trying todo. He stresses that both mental simulation and practical reasoning are necessaryto achieve this kind of understanding, and he labels this account as the dualhypothesis regarding explanation (p. 65).

This hypothesis is reasonable in its own right, but still does not provide anydefinition of what understanding i s i t only suggests what cognitive skills weneed to grasp it. Namely, we need to be able to put ourselves in the shoes ofsomeone else, and then derive the practical consequences of that particular situation,so that we can understand it. But what does it mean to 'understand it' in the firstplace? What is this understanding that should be transferred in an explanationdialogue? In Walton's words, how can understanding increase or decrease invirtue of an explanation? (p. 65).

The answer to these crucial questions is, with respect to the generalsophistication of Walton's analysis, rather vague and disappointing. He suggests


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beyond seeing it as a single inference, called abductive inference, with characteristicpremises and conclusion. What is needed is even to go beyond seeing abductionas a chaining of reasoning with several inferences connected in a sequence. Oneneed is to grasp the ultimate aim of such a sequence by seeing the reasoning asused within a larger framework (p. 179, my emphasis). Here Walton seems partiallycarried away by his own enthusiasm, since the pragmatic context in which abductionoccurs is certainly crucial to assess its validity, as Walton illustrates, but it has·nothing to do with the logical form ofabduction -that is, with the formal definitionof what constitutes abduction in the first place (in Walton's view, the twin argumentschemes on pp. 217-218). To grasp such slippery definition, we have no choice butto struggle with the 'atomic' cases of abduction in single inferential steps, to seewhat is it that differentiates presumed instances of abduction from cases of deduction

and induction-following in Peirce's footsteps, we have to get back to beans. Butthis is precisely what Walton suggests we need to go beyond: as a result, hisdialogue model turns out to be necessary (and perhaps sufficient, once betterspecified) to evaluate abduction, but it is neither necessary nor sufficient to defineabduction.

This is not to say that Walton does not commit to any definition of abduction:of course he does, and repeatedly, when he characterizes abduction as inference tothe best explanation. Since this definition includes an evaluative element, best , wemay be tempted to conclude that, indeed, defining abduction implies evaluating its

merits-so that Walton would be ultimately right in his urge to consider pragmaticfactors to grasp the logical form of abduction . On second thought, it is not so: inabduction, an explanation is considered best because it fits most adequately thefacts known at the moment, and there is nothing intrinsically dialogical in this,unless we want to regard the assessment of competing hypotheses as a kind ofdialogue. The fact that what is considered the best explanation may change overtime, once new information are gathered and critical questions addressed, is certainlyinspiring for evaluating real-life cases of abductive reasoning, but it has no bearingon how we define abduction per se n other words, the definition of abduction asinference to the best explanation remains totally independent from the dialoguemodel that we may (beneficially) use to assess its progress in the context of aprolonged inquiry.

Consider again Peirce's beans: when we conclude by abduction that the handfulof white beans on the table were taken from the all-white bag, is this inferencecogent? Clearly it is not deductively valid, but cogency in everyday reasoning israrely a matter of deductive validity. So the answer i s i t depends. According toWalton, it depends whether or not this argument can answer all the critical

'questions' associated with abductive inference (see pp. 222-228). I put 'answer'and questions in quotes because such questions are really asked and answeredonly when the argument itself becomes matter of debate, to clarify whether thelicensed inference can be accepted, and to what extent. Otherwise, those 'questions'are more perspicuously represented as conditions that must hold, if the argument isto be judged as (more or less) valid. Thus the questioning-answering sessiondescribed by Walton is either i) a consequence of the fact that a given argumentcomes to be debated in its own right, or (ii) a slightly metaphorical way of namingvalidity conditions of an argument scheme. If (i) is the case, Walton's claim that we

need a dialogical model of abduction becomes trivial, because what it really says is


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that we need to account for dialogical features to understand abduction in thecontext of a dialogue. If (ii) is the case, the claim is unwarranted, because as soon aswe see critical questions as validity conditions, the need to conceive abduction asinherently dialogical evaporates: we do not ask questions; we check conditions, sothat our evaluation is certainly critical, defeasible, and subject to change over time,but not necessarily dialogical in any self-evident sense.

All this said, these open problems do not hinder the value of Walton's momentousspeculation on abduction, but rather set the framework for future studies in thesame general direction. This openness to future developments, together withcomprehensive coverage of previous works on the topic, is what makes this bookso fascinating and helpful for any scholar interested in the study of abduction. Inthe words of Walton himself: Although some light has been thrown . . . on the

question of whether there is a third category of logical reasoning other than deductiveand inductive, some key questions have merely been posed with more urgency. Inparticular, basic questions about abductive reasoning remain unanswered. But

some problems have at least been posed that suggest directions to carry the inquiryfurther (p. 105). Indeed, it would be impossible to deny that Walton's book hasshown several promising directions of research for a theory o f abduction. Now,following his example, the inquiry is to be carried further.

otes

In his review of this same book, David Hitchcock suggests a different understanding ofWalton's understanding: According to Walton, explanation is the transmission ofunderstanding: an explanation communicates information that enables its recipient to inferthe thing explained. Thus Walton implicitly equates understanding something with beingable to infer it from information at one's disposal (2005: 1 . If Hitchcock is right in hisreconstruction of Walton's thesis, then my accusation of vagueness is mistaken-but inthat case, Hitchcock's own criticisms have to be answered, since ability to infer theoccurrence of a phenomenon is neither necessary nor sufficient for understanding it; for

example, one can understand why an atom of a radioactive isotope decays without beingable to infer it from the data, and one can infer from hearing thunder that lightning juststruck without understanding why the lightning struck (2005 :2 . In contrast, if Hitchcock'sreconstruction misinterprets Walton's views, my challenge of vagueness still remains to beaddressed.

References

Hitchcock, D. (2005). Review of Douglas N. Walton's Abductive reasoning .Manuscript , ht tp: / /www.humanit ies .mcmaster.ca/-h itchckd/abd uctive. pdf(consulted on 30 September 2005).

Walton, D. N. (1996). Argumentation schemes or presumptive reasoning Mahwah:LEA.

Walton, D. N. (1998). he new dialectic: Conversational contexts o argumentToronto: University of Toronto Press.

FABIO P AGLIERI University o Siena

abductive reasoning_dougla walton

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