The difference between epistemic and metaphysical necessity

Abstract

Philosophers have observed that metaphysical necessity appears to be a true or real or genuine form of necessity while epistemic necessity does not. Similarly, natural necessity appears genuine while deontic necessity does not. But what is it for a form of necessity to be genuine? I defend an account of genuine necessity in explanatory terms. The genuine forms of necessity, I argue, are those that provide what I call necessitarian explanation. I discuss the relationship of necessitarian explanation to ground.

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Notes

  1. 1.

    Some philosophers have preferred to say that epistemic and metaphysical necessity differ in their reality (Edgington 2004, p. 2) or objectivity (Williamson 2016) rather than their genuineness.

  2. 2.

    Many philosophers hold that every a priori proposition is metaphysically necessary. Since metaphysical necessity is genuine, must these philosophers take epistemic necessity, in the sense of a priority, to be genuine as well? No. A form of necessity may fail to be genuine even if every proposition with that form of necessity also possesses a distinct form of necessity that is genuine. The account developed below explains how such a situation is possible.

  3. 3.

    Might we instead take a form of necessity to be genuine just in case it is both mind-independent and sufficient for truth? Since deontic necessity is not sufficient for truth, this proposal correctly counts it as nongenuine. But the proposal mishandles the historical form of necessity (discussed further below). A four-dimensionalist in the mold of Sider (2001), for instance, might understand the claim that a given proposition p is historically necessary as nothing more than the claim that p follows, in a certain sense, from the true propositions about our own time and the times that precede it (cf. Lewis 1986, p. 7). Historical necessity, on this view, will be both mind-independent and sufficient for truth. Yet the four-dimensionalist sees time as akin to space and will no more admit a genuine form of necessity connected to times earlier than our own than she will admit one connected to points of space to her left.

  4. 4.

    Epistemic necessity need not be taken to be relative to a person. It might instead be taken to be relative to something else, such as a group or body of knowledge. See the papers in Egan and Weatherson (2011) for discussion.

  5. 5.

    Again one might think to accommodate the deontic case by taking a form of necessity to be genuine just in case it is both nonrelative and sufficient for truth. But one might agree with Quine (1943, p. 121) that ‘among the various possible senses of the vague adverb “necessarily”, we can single out one—the sense of analytic necessity—according to the following criterion: the result of applying “necessarily” to a statement is true if, and only if, the original statement is analytic.’ And one might have a purely epistemic conception of analyticity on which a statement is analytic just in case ‘grasp of its meaning alone suffices for justified belief in its truth’ (Boghossian 1996, p. 363). Analytic necessity, so understood, is both nonrelative and sufficient for truth. Yet it is no more genuine than epistemic necessity.

  6. 6.

    Accounts of fundamentality have been developed by Fine (2001), Schaffer (2009), Jenkins (2011), Sider (2011), Wilson (2014) and Raven (2016).

  7. 7.

    Thus Barnes and Cameron (2011, p. 2) take the open future theorist to hold that ‘there are multiple genuinely possible ways our history could go’.

  8. 8.

    Although Lange himself does not think historical necessity is genuine (p. 211), I take it that any reasonable account of genuineness should at least be compatible with the doctrine of the open future.

  9. 9.

    Kment (2014) develops an account of genuine (or in his terms, ‘ontic’) necessity that requires genuine forms of necessity to be stable in something very much like Lange’s sense (p. 24; see also Kment 2015, p. 524). The doctrine of the open future poses a difficulty for Kment’s account as well.

  10. 10.

    I am grateful to an anonymous referee for calling my attention to the deflationary view.

  11. 11.

    For discussion of ground see Fine (2001), Schaffer (2009) and Rosen (2010), among others. I take no position on what Raven (2015) calls the unionist/separatist debate.

  12. 12.

    Pace Schaffer (2012).

  13. 13.

    Fine (2012) argues that this condition is too strong. His logic of ground contains a disjunction elimination rule that supports only a weaker condition on the grounds of disjunctions. Our argument can be modified to accommodate Fine’s view.

  14. 14.

    Pace Leuenberger (2014) and Skiles (2015).

  15. 15.

    Related notions of explanatory constraint have recently been discussed by Lange (2016) and (in unpublished work) by Michael Bertrand.

  16. 16.

    The explanatory account also correctly classifies as nongenuine the epistemic conception of analytic necessity mentioned in n. 5. It is no part of why bachelors are unmarried, after all, that some epistemic condition holds. This form of necessity (if indeed it is a form of necessity) is nonexplanatory and thus nongenuine.

  17. 17.

    I am grateful to Jennifer Wang for discussion on this point.

  18. 18.

    Cf. Fine (2002, p. 279): ‘I conclude that there are three distinct sources of necessity—the identity of things, the natural order, and the normative order—and that each gives rise to its own peculiar form of necessity.’

  19. 19.

    I will not here address the proposal that natural-necessitarian explanation is a form of causal explanation understood in some other way, as, for instance, in Skow (2014).

  20. 20.

    Cf. Bernstein (2016, p. 24): ‘It is a substantive metaphysical question whether causes always precede their effects.’

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Acknowledgements

My thanks to Selim Berker, Harjit Bhogal, Dave Chalmers, Cian Dorr, Kit Fine, Matthew Hanser, Marc Lange, Kris McDaniel, Carla Merino-Rajme, Jessica Moss, Asya Passinsky, Zee Perry, Gideon Rosen, Erick Sam, Erica Shumener, Ted Sider, Sharon Street, Michael Strevens, Jennifer Wang and to audiences at NYU, Iowa State University, Koç University, Ashoka University and the APA Pacific Division Meeting.

Funding

I am grateful for the support of the John Templeton Foundation and of the Program of Postdoctoral Fellowships at the Universidad Nacional Autónoma de México.

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Correspondence to Martin Glazier.

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Glazier, M. The difference between epistemic and metaphysical necessity. Synthese 198, 1409–1424 (2021). https://doi.org/10.1007/s11229-017-1626-2

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Keywords

  • Necessity
  • Genuine
  • Explanation
  • Ground
  • Open future