Abstract
How should modal reasoning proceed? Here we compare abduction-based and conceiving-based modal epistemologies, and argue that an abduction-based approach is preferable, and by a wide margin.
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Notes
Those who prefer talk of knowledge, warrant, entitlement, or some other kind of epistemic goodness can substitute accordingly for our talk of justification in what follows, mutatis mutandis.
Some advocates of a conceiving-based modal epistemology deny that inconceivability is a guide to impossibility. Lightner (1997) offers reasons to think that, while Hume took conceivability to be a guide to possibility, he did not take inconceivability (unless indicative of a contradiction) to be a guide to impossibility. Chalmers (2002) suggests that there might be cases of ‘open inconceivabilities’, where a target situation S cannot be positively conceived (say, because involving properties “that simply cannot be positively conceived at all”) but where, insofar as there is no apparent contradiction in S, S’s modal status remains open. These are versions of a conceiving-based modal epistemology provided that they are not conjoined with the further claim that we can know the truth-value of some open inconceivability. Neither Hume nor Chalmers advance that further claim. We could reformulate a conceiving-based modal epistemology accordingly to hold, using the case of zombies for illustration, that we should take the claim zombies are possible to be true if zombies are conceivable, and either false or unknowable if zombies are inconceivable (where the choice between the claim’s being false or instead unknowable would depend on whether the claim meets a sufficient condition for being an open inconceivability). We continue to work with the simpler view that we describe in the main text because shifting to the more complex view described here would not impact the dialectic, except to further support an abduction-based modal epistemology (by undesirably further restricting the range of justified modal beliefs that a conceiving-based modal epistemology can deliver).
A belief-forming method is a priori if it can eventuate in justified beliefs that cannot be justified via (relevant kinds of) experience. See, e.g., Bonjour (1998), Casullo (2001, 2003), Russell (2017) and Biggs and Wilson (2017b) for further discussion of which kinds of experience are relevant to assessing whether a given belief-forming method is a priori; we follow standard practice in assuming that perceptual experience is relevant to this determination, but certain kinds of cognitive experience, including experience associated with coming to possess concepts associated with a given belief, and experience of reasoning, are not.
Might abduction and conceiving each be ultimate arbiters of modal disputes? Not if they ever deliver different results as directed at the same claim. For in any such case, and given that we should believe whatever results an ultimate arbiter delivers, we would be forced to believe a contradiction, which even if possible is not advisable. This much is compatible with conceiving and abduction both being ultimate arbiters in non-overlapping domains (e.g., pertaining to abstracta and concreta); but such a view is antecedently unsystematic, and in any case (as we will substantiate in what follows) these forms of modal justification frequently do target the same claims and at least sometimes deliver different results; correspondingly, there can be only one ultimate arbiter. For further discussion of each modal epistemology see Biggs (2011); Biggs and Wilson (2017b).
Why do we doubt the familiar claim that the epistemology of actualized possibilities is comparatively easy? Suppose that Sam and Fatema attend Ping’s funeral. Sam takes her seeing Ping’s corpse to be her seeing Ping. She then deduces from the belief that she sees Ping (conjoined with her belief that corpses lack minds) that a person can survive without their mind. But surely, it’s not that easy to falsify psychological theories of personal identity! And what are we to do when Fatema denies Sam’s observational premise, claiming to see Ping’s corpse but not Ping himself? The lesson: perception cannot establish the premise about actuality that Sam needs in order to infer her modal conclusion. Although we cannot substantiate the following suspicion here (though see footnote 9 for relevant further discussion), we suspect that most attempts to deploy the deduction-from-perception procedure suffer a similar shortcoming, even if we fail to notice this shortcoming when we agree about what we “see”.
The quote in full is as follows:
Conceivability is the only guide to necessity; our concepts, and the intuitions about possibility that derive from them, provide our only grip on modal claims. [...] [I]t’s worth mentioning that modal intuitions—intuitions about what is possible and impossible, which it is the aim of conceivability arguments to reveal—are as important to arguments for reductionism as they are to anti-reductionist claims. Again, reductionism entails that it’s impossible for the reduced property to vary independently of the reducing property. Since a claim of impossibility cannot be established by considering the actual world alone (though of course it can be refuted in this way), the reductionist must consider whether certain non-actual scenarios are possible. And the only way to determine this is to use the method of conceivability. (Gertler 2006, p. 205)
One might wonder whether Kant is wrong here: can’t enumerative induction provide a basis for ampliative modal knowledge (as per Leon 2017), such that, though Sam is on time for work today, she can infer that she could have been late from the premise that today is relevantly similar to the many days on which she has been late in the past? As we see it, however, this induction presumes, rather than establishes, that Sam is merely contingently on time for work; for it presumes, rather than establishes, that the person who arrived late in the past was Sam, notwithstanding their lateness. It is the presumption rather than enumerative induction that is doing the heavy epistemic lifting here. Nor is is clear that the presumption at issue is justified through (memory plus) observation, as premises in enumerative inductions often are; for we must judge, rather than observe, that the person that arrived late on previous occasions is Sam, notwithstanding their lateness. After all, the world would look exactly the same whether it was necessary or rather merely contingent for being Sam that Sam be on time. See footnote 7 for relevant discussion.
Possible exceptions to this claim include Fischer (2016, 2017), Roca-Royes (2017), Leon (2017), Williamson (2013), Hanrahan (2017), Bueno and Shalkowski (2015) and Strohminger and Yli-Vakkuri (2017)—though in our view, these proposals succeed to the extent they do in virtue of appealing to an a priori mode of inference (e.g., abduction), notwithstanding their stated claims to the contrary.
Similar remarks hold even if, following Lewis (1986), one takes the truth of modal claims to reflect goings-on in possible worlds of the same (concrete, individual) type as the actual world: since no one at a given concrete possible world can observe goings-on at other concrete possible worlds, any modal reasoning on this view must proceed in a priori fashion. Nor will appeal to some other metaphysical property (e.g., essentiality) or relation (e.g., identity) as the empirical basis for modal claims help, for the epistemology of any such property or relation will face the same problem. For example, Sam stands at sixty-two inches whether her height is essential or accidental, and were the person inhabiting Sam’s body to grow or shrink, we would judge, not see, that Sam survives (or does not survive) the change; similarly, Mark Twain shares Samuel Clemens’s white hair and mustache, whether the two are identical or are merely spatiotemporally coincident.
Here we have in mind a principle of qualitative ontological parsimony, according to which, ceteris paribus, one should prefer theory T over theory T* if T implies the existence of fewer kinds than T*. Most of what we say applies, and all crucial points apply, mutatis mutandis, to any principle of parsimony or ‘simplicity’, including principles of quantitative ontological parsimony and principles of syntactic simplicity, according to which, ceteris paribus, one should prefer theory T over theory T* if (assuming a syntactic view of theories) T includes fewer logical connectives and sentences than T* or (assuming a semantic view of theories) if T is a more elegant model than T*. See Sober (2015) for further discussion of principles of simplicity.
One might think that the consequent of premise (3) and the conclusion should rather express that Parsimony is at best a contingent mark of truth. This qualification would be superfluous, however, since we are presupposing that Parsimony, as an abductive principle, actually has epistemic value.
Any principle offering such counsel would render observation irrelevant to theorizing, since we could then immediately infer that we should believe that nothing exists, regardless of what we observe!
Compare the reasons set out in Wilson (2015) for thinking that the presence or absence of quiddities—the property equivalents of primitive haecceitistic identities—is irrelevant to the denotations of scientific expressions and the truth values of claims using such expressions.
This point can help make sense of Kant’s view that, as Sober puts it, “reason commands us to assume that the laws of nature are simple” despite the fact that “we have no a priori guarantee that the world is actually simple” (2015, 41). To say that “we have no a priori guarantee that the world is actually simple” is to say that there are worlds where theories that posit fewer kinds tend to be false. To say that “reason commands us to” believe simpler theories is to say that a principle of simplicity has epistemic value.
Even given these two suppositions, it is unclear that Parsimony would tend to lead the inhabitants of the hidden entity world to form false beliefs. Any beliefs about derivative entities that inhabitants form via applications of Parsimony, and which are false due to the presence of the hidden entities at issue, would be the result of false premises about the fundamental, and so, (as per our third response) it would not be Parsimony that leads them to those false beliefs. Accordingly, at most, Parsimony would mislead the inhabitants with respect to beliefs about the fundamental that include a “thats all” clause, and whose truth-value is impacted by the hidden entities. Presumably, that would be a rather small subset of the total number of beliefs that the inhabitants would or could form through Parsimony.
As Chalmers puts it: “There may of course be borderline cases in which its indeterminate whether a concept would refer to a certain object [...] This is no problem: we can allow indeterminacies [of this sort] as we sometimes allow indeterminacies in reference in our own world” (p. 364).
The fact that this bifurcation is compatible with ‘bird’ having a ‘stable primary intension’, understood as a function from worlds considered as actual to extensions (where, on the original suggestion, a world where one first encounters a plane on the ground is considered a different actual world than a world where one encounters a plane in the air) does nothing to alleviate the present concern, since Frank and Gina occupy the same world.
Yet more recently, Chalmers (2014) briefly suggests that he now prefers to retain a fully general scrutability thesis by shifting the indeterminacy at issue to scrutability itself. This new stance is underdeveloped but we see no reason to think that it can improve his position in the present dialectic.
Kant also thinks that we cannot define non-mathematical a priori concepts—the exception being “arbitrarily thought” concepts (i.e., those with entirely stipulated content) (p. 638, A 729). We only address empirical concepts here.
Similarly, Kant says, “when, e.g., water and its properties are under discussion, one will not stop at what is intended by the word “water” but rather advance to experiments, and the word, with the few marks that are attached to it, is to constitute only a designation [i.e., an actually reference-fixing mark] and not a concept of the thing [i.e., a counterfactually stable mark]; thus the putative definition is nothing other than the determination of the word” (p. 638, A 728).
Perhaps if the chef learns that he has often applied vinegar to acetic acids, he will recognize an inconsistency in his dispositions. But conceiving cannot resolve this intrapersonal inconsistency any more than it can resolve the interpersonal inconsistency between a person who is disposed to apply gold only to non-rusty metals and one who is disposed to apply gold to some rusty metal; in neither case does the mere existence of inconsistency reveal which disposition should be rejected.
Attempts to explain modal disagreement within a conceiving-based modal epistemology substantiate this result. Such attempts either suggest that nearly all modal disagreement is merely verbal [as per, e.g., Chalmers (2012), Yablo (1993)] or else embrace abductive principles as relevant to deciding modal disputes. For example, Geirsson (2005) suggests that those who disagree about the extension of a predicate at a scenario should appeal first to consilience, determining whether some background belief generates the dispute, and then to simplicity, among other abductive principles, so that when “factual or other investigation does not settle the differences”, perhaps disputants “should evaluate their scenarios in a similar way as scientists evaluate theories; by looking at such issues as simplicity and, when applicable, predictability in addition to consistency and coherence” (p. 299).
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Acknowledgements
The authors would like to thank David Alexander, Ranpal Dosanjh, Benj Hellie, Antonella Mallozzi, Elanor Taylor, participants at the 2018 APA symposium on inference to the best explanation, and this journal’s anonymous referees, for helpful comments.
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Biggs, S., Wilson, J. Abduction versus conceiving in modal epistemology. Synthese 198 (Suppl 8), 2045–2076 (2021). https://doi.org/10.1007/s11229-019-02117-9
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DOI: https://doi.org/10.1007/s11229-019-02117-9