Starting in his (2003), and in an unpublished draft from around the same time which is not to be cited, Theodore Sider has proposed a theory of necessity de dicto called quasi-conventionalism. The most up to date version can be found in his (2011) and his replies to a symposium on that work. It states necessary and sufficient conditions for a proposition to be necessary, but as we will see, one of the key concepts involved has been left open-ended, so the account is to be regarded as partial. The account is supposed to reduce necessity de dicto to non-modal notions, and to be extendable to de re modality.

What makes Sider’s account so worthy of discussion from my point of view is that it takes what I believe to be an important step forward with respect to the task of giving an account of necessity de dicto. The step forward is that it embodies a certain structure, which my account shares. Abstracting from the details of Sider’s account, the shared structure can be expressed in the form of a schematic analysis as follows:

(Schema) A proposition is necessary iff it is, or is implied by, a proposition which is both true and meets a certain condition C.

(Sider, as we shall see, does not quite use his schema, but his analysis can easily be re-expressed so as to conform to it.)

So, Sider’s view takes, as I will argue, an important step forward. But it also has grave defects. Considering Sider’s view, then - seeing that it instantiates the suggestive and appealing (Schema), and seeing what is wrong with it - offers a nice way of leading up to and motivating my own account, which I will give in the next chapter. (This is not the way I actually arrived at my account, but it could have been.)

Two of SIder’s main starting points seem to be: (i) the desire to find a way of reducing modal concepts to non-modal concepts, and (ii) a hunch that conventionalist theories according to which necessities are true by convention were on to something: roughly, that convention should play some key role in the analysis of necessity. Regarding (i) and the underlying motivation for it, there are two interrelated strands here: one is a relatively theory-neutral feeling that modality is mysterious, or cries out for explanation, but this then plays into the second strand, which is emphasized in his (2011): a desire for an account of the “fundamental nature of reality”, “reality’s fundamental structure” - an account that “carves nature at the joints”.

Sider argues that it was always a mistake to try to account for necessary truth by means of the idea of truth by convention: the idea is of dubious coherence, and in view of necessary a posteriori truths especially, would not seem to line up with the idea of necessary truth anyway. But that doesn’t mean convention can’t play a crucial role in, not truth-making, but necessity-making, or accounting for the necessity of necessary truths. (I make the analogous point, with the meanings or natures of propositions in place of convention, in arguing for my account.) Part of the motivation and attraction of truth by convention theories of necessity, Sider allows, was their promise of shedding light on the epistemology of logic and mathematics. The theory of modality Sider is offering makes no claim to do that. But so what? Who says the place to look for insight into the epistemology of logic and mathematics is in a theory of modality?

In his (2011), Sider labels his account ‘Humean’. Here is his first pass at expressing it there:
To say that a proposition is necessary, according to the Humean, is to say that the proposition is i) true; and ii) of a certain sort. A crude Humean view, for example, would say that a proposition is necessary iff it is either a logical or mathematical truth. What determines the “certain sort” of propositions? Nothing “metaphysically deep”. For the Humean, necessity does not carve at the joints. There are many candidate meanings for ‘necessary’, corresponding to different “certain sorts” our linguistic community might choose. (Sider 2011, p. 269.)

The role of convention in Sider’s account, then, lies in distinguishing this “certain sort” - or “certain sorts” (Sider switches as this point to the plural):
Perhaps the choice of the “certain sorts” is conventional. Convention can do this without purporting to make true the statements of logic or mathematics (or, for that matter, statements to the effect that these truths are necessary), for the choice of the certain sorts is just a choice about what to mean by ‘necessary’. Or perhaps the choice is partly subjective/projective rather than purely conventional. (p. 270.)

As can be seen at the end of this last quote, Sider has some uncertainty about whether the choice of the “certain sorts” is ‘purely conventional’. We will not get deeply into Sider’s ideas of ‘conventional’ and ‘subjective/projective’ here. It is enough for our purposes that the “certain sorts” are, for Sider, ‘not objectively distinguished’ (p. 270). Or again in different words:
The core idea of the Humean account, then, is that necessary truths are truths of certain more or less arbitrarily selected kinds. (p. 271.)

At this point Sider introduces a refinement, and it is this that will allow us to see how Sider’s account embodies (Schema) above:
More carefully: begin with a set of modal axioms and a set of modal rules. Modal axioms are simply certain chosen true sentences; modal rules are certain chosen truth-preserving relations between sets of sentences and sentences. To any chosen modal axioms and rules there corresponds a set of modal theorems: the closure of the set of modal axioms under the rules.[footnote omitted] Any choice of modal axioms and modal rules, and thus of modal theorems, results in a version of Humeanism: to be necessary is to be a modal theorem thus understood.[footnote omitted] (“Modal” axioms, rules, and theorems are so-called because of their role in the Humean theory of modality, but the goal is to characterize them nonmodally; otherwise the theory would fail to be reductive. [...]) (p. 271.)

Then, getting more specific with a preliminary proposal:
A simple version of Humeanism to begin with: the sole modal rule is first‐order logical consequence, and the modal axioms are the mathematical truths. (Logical truths are logical consequences of any propositions whatsoever, and so do not need to be included as modal axioms.) (pp. 271 - 272.)

At this point, we can see how Sider’s account, or type of account, will embody (Schema); his notion of ‘modal axiom’ combines the requirements of truth and being of a “certain sort” (or one of “certain sorts”), and the main point of the ‘modal rules’ seems clearly to be to draw out implications of the axioms. So, separating the truth and “certain sort” requirements again, we can with little or no distortion put Sider’s preferred type of account into (Schema):

(SiderSchema) A proposition is necessary iff it is, or is implied by, a proposition which is both true and is of a more or less arbitrarily selected “certain sort”.

The presence in the account of implication (or something like it) is in my view an important and laudable feature. It is perhaps not sufficiently emphasized by Sider, and has been glossed over in subsequent discussion of his view. For instance, Merricks (2013) glosses Sider’s account as saying that ‘Sider reduces a proposition p’s being necessarily true to: p is true-and-mathematical or true-and-logical or true-and-metaphysical or…’. The importance of implication (or something like it) in (Schema) and views embodying it is made clear in my recent post on my account.

After giving his preliminary version of Humeanism, Sider goes on to consider a series of worries, responding with ‘a combination of refinement and argument’ (p. 272.) He never arrives at a definitive proposal, but aims to develop his strategy sufficiently to justify his general approach.

I think we have already gotten a pretty good sense of Sider’s approach, but I want more or less to complete the exposition of Sider’s approach before, in my next post, moving on to objections, none of which are among the worries Sider considers. So before moving on to objections, I will now briefly convey six further points which emerge in Sider’s responses to the worries.

Logical consequence must be non-modal: Sider wants his account to avoid modal notions, so modal characterizations of logical consequence are out. Remaining options include primitivism about logical consequence, something Sider calls the “best system” account of logical truth (which he describes in section 10.3 of his (2011)) extended to an account of logical consequence, and model theoretic approaches.

Analytic truths added as axioms: Sider holds that analytic truths should come out necessary, and proposes to that end that each analytic truth be added as a modal axiom (p. 274). This move is unapologetically ad hoc. (You might worry, as I do, that some examples of the contingent a priori should count as analytic, in which case not all analyticities are necessities. But perhaps there are different notions of analyticity which may give different results here. In any case let’s set this aside.)

“Metaphysical” statements added as axioms: again, modulo some fuzziness about what it takes for a statement to count as metaphysical - the gloss Sider uses is ‘truths about fundamental and abstract matters’ (p. 275) - true metaphysical statements are to be added as axioms. Again this is unapologetically ad hoc, or treated as a brute fact: ‘What justifies their status as modal axioms? This is just how the concept of necessity works. Such propositions have no further feature that explains their inclusion as modal axioms.’ (p. 275)

A new class of ‘natural kind axioms’: another unapologetically ad hoc addition, this time to accommodate the necessity of natural kind type examples of the necessary a posteriori, e.g. ‘Water is H20’. I refer the reader to (pp. 282 - 283) for details.

Contextual variation of the “outer modality”: it is conventional wisdom that modal talk in the wild should be understood as being about a contextually variable space of possibilities. This is often combined with a picture of an outer, unrestricted space of possibilities which does not vary. Sider suggests, as ‘more attractive’ (p. 281), that even the outer space is contextually variable - that ‘there can be contextual variation both in the modal axioms and the modal rules’ (p. 281).

Family resemblances (maybe): on (p. 288) Sider rehearses his (by now familiar) attitude to necessity thus: ‘Why are logical (or mathematical, or analytic, or …) truths necessary? The Humean’s answer is that this is just how our concept of necessity works.’ But then he turns around and suggests (pp. 288-289) that ‘a Humean need not be quite so flatfooted. [...] [The Humean] resists the idea that there is a single necessary and sufficient condition for being a modal axiom. Nevertheless, she is free to exhibit similarities between various modal axioms, just as one might exhibit similarities between things that fall under our concept of a game, to use Wittgenstein’s example. Doing this would help to show that the Humean concept of necessity is not utterly arbitrary or heterogeneous.’ This no doubt helps the plausibility of Sider’s account in a way, but may also play into the hands of an objector, as we shall soon see.

This completes our exposition of Sider’s theory. In the next post we will consider some objections.

*References*

Merricks, Trenton (2013). Three Comments on Writing the Book of the World. Analysis 73 (4):722-736.
Sider, Theodore (2003). Reductive theories of modality. In Michael J. Loux & Dean W. Zimmerman

Sider, Theodore (2011). Writing the Book of the World. Oxford University Press.

Sider, Theodore (2013). Symposium on Writing the Book of the World. Analysis 73 (4):751-770.