Abstract
Corroborative evidence has a dual function in argument. Primarily, it functions to provide direct evidence supporting the main conclusion. But it also has a secondary, bolstering function which increases the probative value of some other piece of evidence in the argument. This paper argues that the bolstering effect of corroborative evidence is legitimate, and can be explained as counter–rebuttal achieved through inference to the best explanation. A model (argument diagram) of corroborative evidence, representing its structure and operation as a schematic pattern of defeasible argument is also supplied. In addition to explaining the operation and theoretical foundation of corroborative evidence, the model facilitates the correct analysis and guides the evaluation (assessment and critique) of corroborative evidence as it occurs in argument.
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Notes
Schum (1994, pp. 66 ff.) seems also to have adopted this three-part standard for the evaluation of evidence in law, using the criteria relevance, credibility (acceptability) and force (sufficiency).
This form of strengthening can also contribute to the cogency of deductively valid arguments by establishing their soundness.
It should be recognized that the very distinction between linked and convergent arguments (cf. Freeman 1991, pp. 96ff.) is the subject of theoretical debate (primarily due to Goddu 2003, 2007, 2009). Briefly, Goddu (2003, 2007) argues that the standard ways of operationalizing the distinction fail, and (2009) that the “we have no good reason for making the distinction,” even if it could be successfully operationalized, because it is not sufficiently useful to the structural analysis or evaluation of argument. I offer no reply to Goddu’s criticisms, instead relying on an intuitive understanding of the distinction (which seems to have paradigmatic exemplars) and on its continued employment in informal logic, argumentation and epistemology. For a thorough treatment of this topic see Freeman (2011, chaps. 4–6, pp. 89–172).
The expanded standard approach to argument diagramming developed by Freeman (1991, 2011) will be used throughout. See his (2011, chap. 1) for an overview. While the standard approach models many structural aspects of arguments (such as premises, conclusions, and inferential patterns and connections), Freeman’s expanded standard approach is capable of modeling additional argumentative components such as modal qualifiers, rebuttals (defeaters), counter–rebuttals, and counter-considerations. These additional resources permit Freeman’s approach to represent many dialectical aspects of argumentation in argument models.
As used here though, instead of representing individual sentences, nodes in the diagrams represent kinds of sentences differentiated by their functional role in the argument (schema).
Merriam-Webster’s Dictionary of Law (1996, p. 172); cf. Findlaw Legal Dictionary entry for “evidence”: http://dictionary.findlaw.com/definition/evidence.html; cf. “evidence which strengthens, adds to, or confirms already existing evidence” (The Free Legal Dictionary, http://legal-dictionary.thefreedictionary.com/corroborating+evidence); “evidence that strengthens, adds to, authenticates, or confirms already existing evidence” (Nolo’s Plain-English Law Dictionary, http://www.nolo.com/dictionary/corroborating-evidence-term.html); “supplementary evidence that tends to strengthen or confirm the initial evidence” (MyLawTerms.com, http://www.mylawterms.com/Corroborating-evidence.html). All pages accessed July 12, 2011.
Importantly, incorporating modal qualifiers into the diagramming method allows the strength of the inferential link in the argument to be variegated. This might be done qualitatively, in terms of degrees of plausibility, by using a spectrum of qualitative qualifiers (e.g., perhaps, plausibly, on balance of considerations, probably, presumptively, practically certain, to a moral certainty, beyond a reasonable doubt, necessarily, etc.) each of which could be operationalized in logical, epistemic, or dialogic terms according to the normative framework employed. Alternately, following Godden (2005, p. 171) this spectrum might be articulated in terms of the possibility or relative likelihood of potential counter examples (e.g., no counter-example is logically possible, no counter-example is nomologically possible, as a matter of fact there are no counter-examples, any counter-example would be highly improbable, any counter-example would be less likely than the combined likelihood of the premises, any counter-example would be less likely than the conclusion, no counter-examples are known, no counter-examples are among our present beliefs). Alternately, modal qualifiers might be articulated quantitatively in terms of the degree of probability conveyed by the inference.
Yet, the expanded standard approach is limited in the way it handles qualifiers. A fully probabilistic account attaches probability values to individual claims, or nodes within the diagram. By contrast, Freeman (1991, chap. 5; 2011, pp. 18 ff.) argues that, in the spirit of Toulmin, “qualifiers indicate the strength conferred by the warrant on the step from data to claim” (Freeman 2011, p. 18), and hence are best modeled as attached to lines in the diagram. As such, although not ideal, degrees of credence ascribed or attached to individual claims in the argument must be represented as part of the content of the claims themselves.
It might seem as though the premises in Fig. 3 concerning motive, means, and opportunity should be linked together rather than depicted as separate and convergent, since on their own they fail to provide sufficient support to establish guilt. At best, the premises seem individually necessary and jointly sufficient. As such, denying any one of them would be sufficient to rebut the entire argument. Thus the premises are interdependent rather than independent. While this is true, the arrows in the diagram do not represent lines of sufficient support, but merely lines of support—that is, they mark reasons, not sufficient reasons. While this technique comes with the cost just noted, the benefit is that the independence of the individual reasons is correctly represented; disproving motive does not discount opportunity. Also, while in this example there seems no other way to achieve sufficiency, in other cases there might be alternative combinations of primary reasons capable of sufficiently supporting a conclusion.
Such a technique is entirely foreign to Freeman’s expanded standard approach of argument diagramming, and is proposed here for illustrative purposes only.
At least one argument against the epistemic legitimacy of the bolstering effect of corroborative evidence is that it gives rise to the fallacy of double counting, whereby the probative force of some piece of evidence is overvalued by counting it twice (Redmayne 2000). In previous work (Godden 2010), I have offered a rebuttal of Redmayne’s arguments that corroboration involves the fallacious double counting of evidence.
Justification, in the sense just used, is something (call it a “justifier”) on the basis of which the acceptability, or epistemic status, of a claim is founded.
Justification, standardly understood, has both externalist (or objective) and internalist (or subjective) aspects, each of which contribute to its epistemic desirability. Objectively, justification must serve to reliably connect belief to truth somehow, such that justified beliefs are more likely to be true than unjustified ones. Subjectively, justification must somehow bestow entitlement upon justified believers, perhaps by underwriting claims to rationally, blamelessly, or responsibly held belief. Because of this last aspect, justification must be accessible to justified believers. Without the first aspect, justification is not worth having even if it is accessible; without the latter justification cannot serve as a guide or norm for thought and action. Importantly, these two aspects of justification can come apart. One can blamelessly hold an objectively faulty or unreliably-formed belief; and one can irresponsibly hold an objectively good or reliably-formed belief. Internalists are inclined to forgive the former and condemn the latter, while externalists are inclined to reject the former and endorse the latter.
The challenge concerning primitively justified claims goes as follows. Given that a justifier is something on the basis of which the acceptability of a claim is founded, unless justification can be circular, justifiers must be something other than the claim being justified. Now, for a claim, P*, to be primitively justified (sometimes called a “basic belief”) it must be non-inferentially justified—there must be no other claim on the basis of which P* is justified. Yet, as BonJour (1978, 1985, p. 31) observes, the selection of some beliefs rather than others as basic gives rise to the problem of the criterion: on what basis does the foundationalist select these (kinds of) beliefs, P*, rather than those, P†, as primitively justified? Answering the problem of the criterion—at least in a non-arbitrary way—seems to require justifying one’s answer. Yet, any account that one gives, involving any claim other than P* in an effort to demonstrate (or even merely illuminate) that P* is primitively justified is prima facie evidence that P* is not primitively justified.
Some have attempted to navigate the horns of this dilemma by pointing to putatively non-propositional (i.e., conceptually non-contentful) items as primitive justifiers. Externalists point to a special set of justificatory facts in the world, while givinists point to a special set of justificatory, non-cognitive items of consciousness (or conscious events), such as the experience of being appeared to in a certain way. The claim is that, while these facts or appearances themselves stand in no need of justification, they can effectively be justifiers, i.e., a source of justification for basic beliefs.
Problematically, as Bonjour (1978) observes, neither strategy satisfactorily answers or avoids the problem of the criterion. Rather, the foundationalist must give some account of why these, rather than those, factual properties or conscious characteristics are proper grounds for primitive justification.
Further, against the givinist, BonJour (1978) presents the following paradox of non-cognitivism: to the extent that items of consciousness are conceptually non-contentful they might stand in no need of justification, but it is not at all clear how they can confer justification to conceptually contentful items such as propositions or beliefs. (To confer justification at least involves making the truth of something more likely that it would have been otherwise. Yet non-cognitive items lack the semantic properties required to do this.) On the other hand, to the extent that items of consciousness are conceptually contentful, and thereby capable of conferring justification on other conceptually contentful items, it is not clear why they do not, themselves, stand in need of justification (cf. Sosa (1980, §4, pp. 6–9) for a critical discussion of this last argument.)
Against the externalist BonJour (1978) notes that, to the extent that a believer is unaware of the relevant justificatory facts, she is epistemically irresponsible, and hence lacks justification in that, internalist, sense. Yet, to the extent that a believer is aware of the relevant facts, she seems to have a belief whose justification may be sought, challenged, and given.
Developing the externalist strategy Alston (1989, p. 82) distinguishes between being justified and justifying, stressing that one may be justified (understood as an epistemic state a subject can be in) without ever producing the justification or otherwise doing anything to demonstrate one’s justification (understood as an epistemic activity one can engage in). Thus, the claim is that one can be justified in accepting some prospective P* without having to do anything to demonstrate this. This distinction is thought to relieve the force of the challenges just given.
Importantly, though, the distinction between being and doing justification does not uniquely apply to primatively justified claims, but can apply equally to claims whose justification is derivative. A more articulated set of distinctions begins with one between being justified and having justification, where the latter stresses something that one possesses such that they could produce it if called upon to do so. A second distinction can then be drawn concerning the activity of justifying—specifically concerning the nature of what one produces when one gives one’s justification. Here what is to be distinguished is an explanation of one’s justification from the justification itself. The question here is whether, in the activity of justifying, one produces a justification (which they possessed all along), or whether what is produced is merely an explanation of a justification (which itself is never really produced, and perhaps never really possessed). In the latter case, one does not “give” one’s justification as it is something which, in an important sense, cannot be shared e.g., in the way we could share the same reasons for believing something. Rather, either one is justified (and perhaps has the justifiers), or one isn’t (and doesn’t).
In the end, though, the distinctions between being justified and having justification, and giving a justification or merely an explanation of a justification, do not satisfactorily answer the challenge on the table. To the extent that the basic believer possesses a justification, they are epistemically entitled to their belief, yet it is not basic, since they are in a position to produce some justification or reason for the belief. Alternately, to the extent that the basic believer does not possess any justification, their belief may be properly basic, but it is not clear that they are justified—at least not in any traditional, internalist sense since they cannot demonstrate (even to themselves!) any epistemic entitlement to their belief. At the very least, then, such believers are epistemically irresponsible. The only way to avoid this challenge is to concede that justification, in basic cases anyway, can be non-viciously circular or not required at all. As BonJour (1978, p. 8) writes, such a move abandons the traditional conceptions of justification and knowledge entirely: “it constitutes a solution to the regress problem or any problem arising out of the traditional conception of knowledge only in the radical and relatively uninteresting sense that to reject that conception is also to reject the problem arising out of it.”
The distinction between giving justifications and giving explanations of justifications is, similarly, of little help. Seemingly, if “telling a story about how or why a claim is intrinsically justified” is understood as explaining why one is justified, no problem arises for the foundationalist here. Offering an explanation why something is the case rather than not is not to give a reason that it is the case. Yet, insofar as such stories are offered in response to criticisms of the view held, or challenges of entitlement to the view held, they appear prima facie to be justifications, not explanations of intrinsic justifications. Again, if the justification were intrinsic and intrinsically apparent, no story would be required. Further, such stories tend to cite evidential factors neither identical with nor equivalent to the supposedly self-justified claim, again indicating that claims other than the basic belief itself are doing justificatory work.
The point here is not to resolve, or even to take a stand on, any of these well-known problems which sit on the ground floor of any foundationalism. Rather it is to explain one historical motivation for coherence as a source of justification.
As van Cleeve (2005) observes, there remains the issue of whether each independent source must have some positive initial credibility which can be amplified by coherence, or whether coherence alone can produce credibility where initially there was none. A moderate foundationalism holds that coherence has an ampliative but not a productive function.
Walton and Reed (2008, pp. 544 ff; cf. Walton 2008, pp. 300 ff.) model the corroboration scheme as a linking of all the corroborating claims supporting the conclusion that there is corroborative evidence for some claim. This schematic argument then acts in a convergent manner together with each individual premise in directly supporting the main conclusion.
Such an understanding of the nature of warrant might encourage the view that, in any argument there is only one warrant which (implicitly) expresses the consequence relation held to obtain between all of the argument’s premises (taken together) and its conclusion. There is certainly something to the idea that we, in the end, take our judgements to be based on the sum total of our evidence. As a corollary, all argument diagrams should then have only one arrow (representing the warrant). Such a position seems to accommodate Goddu’s recent criticisms (op. cit.) of the linked/convergent distinction.
To my thinking, though, such a position does not sit well with the fact that arguments can involve several different reasons, even several different, individually sufficient reasons.
Yet, even if such a view is accepted, it remains important to understand the contribution made by individual premises and reasons to arguments. For example, a theory of evidence or argument should be able to explain the structural and evaluative impact of the defeat of a single premise or reason in an argument, and similarly when a single premise or reason is added. On a ‘single warrant’ model of argument, such changes merely occasion the drawing of a new warrant which must be evaluated from scratch. Rather than this, I suggest, it is preferable to try to understand, explain and represent the probative contributions of individual premises and reasons in an argument. Doing so, I suggest, involves tracking, mapping and explaining their evidentiary interrelationships with one another, as well as their individual acceptability, since these structural features help to explain the relevance and probative force of premises (as they contribute to reasons), and reasons (in their contributions to complex argumentation).
Toulmin (1958/2003, chap. 3) explains a warrant as a having the form of a general statement relating the data and claim of an argument, and which articulates the rule or principle which a reasoner relies upon in inferring the claim from the data. Recent work on the nature of warrants (Pinto 2006, 2007, 2009) and on the nature of non-logical consequence (Hitchcock 2009, 2011) represents significant theoretical advancements on this general idea. According to Hitchcock (2011, p. 224):
On the elaborated and expanded account, a conclusion follows from given premisses if and only if an acceptable counterfactual-supporting covering generalization of the argument rules out, either definitively or with some modal qualification, simultaneous acceptability of the premisses and non-acceptability of the conclusion, even though it does not rule out acceptability of the premisses and does not require acceptability of the conclusion independently of the premisses.
The treatment of a warrant as a warranting conditional employed in this paper is intended a simplification of these theories—one that captures their core idea without distorting or contradicting any of their details. As such, it should be recognized warranting conditionals might have the form of covering generalizations of some sort, whether universal or defeasible, complete with any quantification, qualification, specification, modality, or counterfactuality as required.
Following Hitchcock (2009, 2011) warrants are more accurately represented as covering generalizations. For simplicity and convenience, they are here treated as a special kind of conditional: a warranting conditional. The differences between these two ways of representing warrants do not bear on the present argument.
Warranting conditionals of the form ‘P → C’ are related to, and can be markers for (defeasible) consequence relations of the form ‘P |~C’ (read as “C follows from P”), just as the hook (material implication) is related to the turnstile (entailment). That is, there is some operator ‘→’ such that the (defeasible) consequence relation ‘P |~C’ is satisfied if and only if ‘P → C’ is true under some appropriate modal qualification.
We might just as well have considered an example inference from observation of the reading or output of some measuring instrument, in which case we would have something like the following scheme: Instrument I reads that P, therefore, presumptively P, unless D (where D indicates some defeater, e.g., the instrument is defective, malfunctioning, or miscalibrated).
Having noted that it is our practice to routinely rely on the testimony of individuals and the reports of instruments without making specific, case-by-case credibility or reliability checks, the account to follow is neutral with respect to the presumptive (or primitive) acceptability of these kinds of evidence. This note explains how this neutrality is achieved.
In the case of testimony, for example, Lackey (2006, pp. 4–6; 2008, chap. 5) distinguishes between reductionist and non-reductionist accounts of the acceptability of testimony. On the reductionist view, the acceptability of testimony (even if taken to be defeasible) is not basic, but rather depends upon the prior acceptability of non-testimonial reasons supporting the testifier’s credibility. As an example, Lackey (2006) quotes Ficker (1995, p. 404) who writes “My reliance on a particular piece of testimony reduces locally just if I have adequate grounds to take my informant to be trustworthy on this occasion independently of accepting as true her very utterance.” By contrast, non-reductionists hold that testimony is a primitive kind of evidence whose acceptability (if only presumptive) is basic and not based on the prior acceptability of evidence from other sources. As an example, Lackey (2008, p. 156) quotes Audi (1998, p. 142) who claims that “gaining testimonially grounded knowledge normally requires only having no reason for doubt about the credibility of the attestor.” Each of these positions has adherents (cf. Lackey 2006, pp. 20–21 fn. 10 and 16). Lackey herself (2008) recommends a hybrid, or dualist, position.
As an aside, and despite our routine practices, I have reservations about the non-reductionist’s acceptance policy for testimony, which has been described as follows:
so long as there are no relevant defeaters, hearers can justifiedly accept the assertions of speakers merely on the basis of a speaker’s testimony. Otherwise put, so long as there is no available evidence against accepting a speaker’s report, the hearer has no positive epistemic work to do in order to justifiedly accept the testimony in question. (Lackey 2006, p. 4)
In my view, there can be cases where one has no available evidence against a speaker’s report, and yet still should not accept it merely on the basis of the their say-so. Consider any claim, P, about which one lacks (sufficient) evidence one way or another, but about which one knows, could reasonably anticipate, or might just suspect, there is reasonable disagreement. Suppose I now come upon some speaker, S1, who attests that P. According to the non-reductionist, lacking any evidence to the contrary, I should accept P on the basis of S1’s testimony. Now, suppose I later chance to meet S2 who assures me that ~P. Having accepted P uncritically on the basis of S1’s say-so, I now have evidence that counts against S2’s credibility—namely, S1’s testimony (if not my own belief that P formed on the basis of it). Thus, the non-reductionist advises that I not accept ~P on the basis of S2’s say-so. As such, local non-reductionism prescribes accepting uncritically only the first piece of controversial testimony one is confronted with about some issue; yet, typically, the order in which one receives information from the world—including witnesses—is only a matter of chance and is entirely epistemically irrelevant. Clearly, such a policy is not rational. And on two counts: (1) it mistakenly advises acceptance of claims on epistemically irrelevant grounds (it is only a matter of chance that I now accept P rather than ~P), and (2) it mistakenly treats attestors differently for epistemically irrelevant reasons (it is only a matter of chance that I accept S1 at his word and not S2 at hers). (The argument offered here parallels Walton and Godden’s (2005, p. 440) rejection of trust over suspicion as a policy for accepting or challenging (declining to accept) assertions made by an interlocutor in argumentation. Roughly, suspicion advises committing only to interlocutors’ assertions that are either already among one’s present commitments or are logical consequences of them. Trust, by contrast recommends accepting all interlocutor’s assertions which are not contradicted by one’s present commitments. While suspicion is clearly too stringent, following the policy of trust leads to the problems just described.)
Despite my reservations about a blanket local non-reductionism of testimony, the account offered here is neutral between these accounts. The neutrality can be achieved by employing Gordon, Prakken and Walton’s (2007) distinction between defeating conditions (which they approach through the lens of critical questions) that function as assumptions and those which function as exceptions. Assumptions mark defeating conditions which work like normal premises in this way: proponents bear the burden of proof to show the assumption holds (i.e., the defeating condition does not obtain) if they are challenged. Here, merely raising the possibility of the defeater is sufficient to rebut the initial argument, and it is the responsibility of the initial proponent to rebut the defeater. Exceptions, by contrast, are those defeating conditions which are presumed not to hold, such that challengers raising an excepting condition as a rebuttal bear the burden of proof of showing the excepting condition to obtain. Here, merely pointing to the possibility of the defeater is not sufficient to rebut the initial argument; rather the objector must also show that the defeating condition actually obtains. Reductionist accounts of testimony treat credibility as an assumption, while non-reductionist accounts treat it as an exception. Using this terminology, I take no position here on whether the credibility of the attestor is an assumed or exceptional defeater to arguments from testimony.
The point here is not that corroboration brings testimony or evidence from the realm of the incredible to the realm of the credible, although this can happen (see example in the next note). Rather, the point is that corroboration bolsters the credibility or probative worth of existing but defeasible evidence.
To pick a real-life example: readers may recall the grisly murder of Lin Jun by Luka Rocco Magnotta which occurred in Montreal, Canada in late May, 2012. The murder was especially gruesome as it involved the dismemberment of the victim; as well a video of the murder was made by the killer and posted online.
The murder was discovered by authorities on Tuesday, May 29, when dismembered body parts of the victim were found to have been mailed to the offices of the Liberal and Conservative political parties in Ottawa, Canada. That same day, the victim’s torso was discovered in a suitcase left in the garbage behind an apartment building in Montreal.
Importantly, the video of the murder was uploaded to the internet on Friday, May 25, where it was quickly discovered by members of the online community, at least one of whom, Roger Renville, a Montana lawyer, repeatedly attempted to report the footage of the murder to authorities. Their response is instructive to our present considerations.
Renville’s story was reported by the Canadian Broadcasting Corporation’s daily radio news interview program “As It Happens” (Thursday, May 31, 2012) where he was interviewed by host Carol Off. The interview may be heard at the following URL: http://www.cbc.ca/asithappens/episode/2012/05/31/the-thursday-edition-31/ [November 15, 2012]
As Renville describes to Off: he discovered the online video on Saturday, May 26, and immediately tried to report it to local and federal US authorities as well as news outlets. In each case his report was disregarded. On Sunday, May 27, having discovered a connection to Toronto, Canada, Renville attempted to report the video to Toronto police. He describes his attempt in this way: “I admitted [to the police] at the outset that this was going to be a strange report—very, very unusual—and I stated that I’d seen a video online of a murder and dismemberment.” In response, Renville reports that he was told by the officer taking the report, “that what I was seeing was probably fake, that it was probably special effects, that special effects were very good, and could make the fake look real. And I was told that it did not make sense what I was saying; it did not make sense that a killer would film himself committing his crime and then post it on the internet.” Indeed, the Toronto police were not even interested in seeing the video for themselves, and declined to be provided with its URL.
Renville actually gave up trying to report the crime until the evening of Tuesday, May 29, when he learned of the body parts having been discovered in the mail. At that point he again attempted to contact authorities, this time the Royal Canadian Mounted Police (RCMP), Canada’s federal police service, in Ottawa. As Renville describes it, the officer to whom he made the report “at first wasn’t really interested to hear that there was a video of a murder online, but when I pointed out that it … included dismemberment she got more interested. And then … I said, ‘You know, I also have his name.’ And she said, ‘You have his name?’ And I started to spell out his name. Half-way through she interrupted me and got excited and she put me on hold for five minutes, and when she came back she was very excited and she said something to the effect that ‘We are looking for this man.’”
For our purposes, what is interesting about this case is that Renville’s initial reports were treated as incredible by everyone whom he contacted. And their disregarding of his testimony was subsequently defended as reasonable. In the course of the interview, host Carol Off reports that “the Toronto police have issued a statement just now saying that the call taker—the person who took [Renville’s] call—they say, acted reasonably given the information that had been provided.” And, Renville himself agrees, saying “That’s hard for me to take issue with. You know, one thing I’ve been clear about is that it was a very, very strange story that I tried to share. It’s only the fact that we now have proof—that we are now having bloody packages arriving in the mail, that’s causing anybody to take it seriously.” That is, while Renville’s testimony was initially—and reasonably—received as incredible, once it was corroborated by other known evidence, such as the body parts found in the mail or the name of the suspect, the credibility of his testimony was re-evaluated and deemed to be trustworthy.
How did this transformation occur? What about the corroborating evidence made this revaluation of the Renville’s credibility rational? According to the theory on offer, the corroborating evidence pointed to the truth of Renville’s testimony as the best explanation of his giving it—rather than, say, the alternative explanation of the Toronto police officer, that the video had been faked since criminals tend not to post self-incriminating evidence on the internet. In this case, that alternative explanation represents a defeater to any argument relying on Renville’s testimony, by explaining the fact of the evidence without granting the truth of what it is putatively evidence of. Thus, to whatever extent this alternative explanation is more plausible than his testimony’s actually being credible, arguments relying on his testimony will be defeated. Importantly, although the corroborating evidence is not, ceteris paribus, consistent with the alternative, defeating explanation, it does not directly rebut or undermine it either. What it does do, though, is point directly to the truth of what Renville testified to. In doing this, the corroborating evidence directly supports one possible explanation of Renville’s testimony—namely that he said what he did because it’s true. And, having direct and independent evidence for the truth of what was said as the best available explanation of why it was said, we now have a reason to count the testimony as credible in a way that we previously did not.
See Coady (1992, chap. 10) on when such skepticism is rational. Coady notes, e.g., that the mere antecedent improbability of what is reported is an insufficient warrant for skepticism (p. 180), as is its failure to conform to our experience of things (pp. 181–189). On the other hand, Coady claims that, though something’s being contrary to the laws of nature does warrant skepticism (p. 179), it may also be warranted in cases that conform to the laws of nature yet do not cohere well with our “general framework of knowledge” (p. 189). Finally, Coady (1992, pp. 189–191) argues, though without explaining how or why, that the mutual corroboration of a number different witnesses can transform testimony that is, by itself, incredible into something that “need not be rejected” (p. 189). In doing this, Coady rejects Hume’s anti-corroboration principle that: “a fact incredible in itself, acquires not the smallest accession of probability by the accumulation of testimony” (as quoted by Coady 1992, p. 182).
These possible worlds represent alternative scenarios or states of affairs which can be contemplated, and the (relative) likelihood of which can be judged by rational agents. The notation denotes different possible worlds according to a set (incomplete, of course) of propositions taken to hold true in them. The probability considered here is subjective—that is, it represents the likelihood that the judging subject ascribes to the different possible worlds and the claims in them. Of course, these judgements of (relative) probability are be made while taking a whole host of claims, representing the knowledge-state or background beliefs of the agent, as true. Although these background beliefs do comprise the context in which the agent ascribed probability and judges the relative likelihood of possible worlds, they are not included in the list defining the possible world itself.
The relative proximity of possible worlds can be understood as follows. Possible worlds can be judged closer or further away from the actual world (or the world of the agent’s background beliefs) according to a principle of conservatism. The fewer and least severe the adjustments (redistributions of truth or probability over the other propositions in the set) required to accommodate (i.e., retain consistency with) the relevant change (e.g., introduction of a new proposition to the set, or removal of one from the set) the closer the possible world under consideration is to some target (e.g., the actual world). Given that we take the world we believe in to be actual, we take the claims that describe it to be true. Conservatism, then, serves as a subjective measure of the likelihood of some contemplated change. The more extensive or drastic the revisions required in a web of belief to accommodate some change, the less likely it should be taken to be.
The point here is this: the weakest rebutters sufficient to defeat to some initial inference will be those that make the least overall change to the other claims holding in the world. Thus, these defeaters will occur in relatively closer possible worlds than defeaters that require more significant adjustments. Further, according to the subjective measure of likelihood just described, the defeaters in the closest possible worlds will also be deemed most likely.
Thus, corroboration shows a belief, C, based on some initial reason, R1, to be sensitive in a way that it was not prior to corroboration. A subject’s, S’s, belief, C, is sensitive if and only if, in the nearest (or nearby) possible world(s) to the actual one where C does not obtain, S would not believe that C (Nozick 1981; Dretske 1971; for an overview see Becker and Black 2012). So long as it is granted that sensitive beliefs, even defeasibly sensitive ones, are epistemically stronger than insensitive ones, then corroboration strengthens the probative value of beliefs by making them sensitive. While not advanced here, this line of reasoning is developed in subsequent work by the author (Godden 2012, 2013).
Circumstances require that a minor modification be made to Freeman’s expanded standard approach to diagramming. Normally, rebuttal conditions would attach to the modal qualifier. Yet, to specify which defeaters are being counter–rebutted, rebuttal conditions are here depicted as attaching to individual lines of reasoning within the convergent argument structure.
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Acknowledgments
I thank John Jackson and Jean Wagemans for their instructive comments on the presented version of the paper, as well as Douglas Walton and David Hitchcock for their constructive correspondence on earlier and revised versions of the paper. Also, thanks are due to Dale Miller for his perceptive and instructive comments on other work that has informed this paper. Most importantly, the account proposed in the paper has been significantly and objectively improved by incorporating the suggestions and responding to the comments made by Argumentation’s anonymous reviewers, to whom I offer my sincere thanks.
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David Godden Modeling Corroborative Evidence: Inference to the Best Explanation as Counter–Rebuttal. Argumentation 28, 187–220 (2014). https://doi.org/10.1007/s10503-013-9308-9
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DOI: https://doi.org/10.1007/s10503-013-9308-9