Monthly Archives: June 2009

Justifying scoring rules

In connection to this paper, I’ve been thinking some more about what grounds we might have for saying substantive things about how “scoring rules” should behave.

Quick background. Scoring rules rank either credences in a single proposition, or whole credence functions (depending on your choice of poison) against the actual truth values. For now, let’s concentrate on the single-proposition case. In the context we’re interested in, they’re meant to measure “how (in)accurate” the credences are. I’ll assume that scoring rules take the form s(x,y), where x is the credence, and y the truth value of the salient proposition (1 for truth, 0 for falsity). You’d naturally expect a minimal constraint to be:

(Minimal 1)  s(1,1)=s(0,0)=1; s(0,1)=s(1,0)=0.

(Minimal 2) s is a monotone increasing function in x when y=1. s is a monotone decreasing function in x when y=0.

Basically, this just says that credences 1 and 0 are maximally and minimally accurate, and you never decrease in accuracy by moving closer to the truth value.

But to make arguments from scoring rules for probabilism run, we need a lot more structure. Where do we get it from?

There’s a prior question: what’s the nature of a scoring rule in the first place? There’re a couple of thoughts to have here. One is that scoring rules are *preferences* of agents. Different agents can have different scoring rules, and the relevant preference-ordering aims to capture the subjective value the agent attaches to having *accurate* credences.

Now, various hedges are needed at this point. Maybe having certain credences make you feel warm and fuzzy, and you prefer to have those feelings no matter what. We need to distill that stuff out. Moreover, maybe you value having particular credences in certain situations  because of their instrumental value—e.g. enabling you indirectly to get lots of warm fuzzy stuff. One strong thesis about scoring rules is that they give the *intrinsic* value that the agent attaches to a certain credence/truth value state of affairs—her preferences given that alethic accuracy is all she cares about. However tricky the details of this are to spell out, the general story about what the scoring rule aim to describe is pretty clear—part of the preferences of individual agents.

A different kind of view would have it that the scoring rule describes a more objective beast: facts about which credences are better than which others (as far as accuracy goes). Presumably, if there are such betterness facts, this’ll provide a standard for assessing people’s alethic preferences in the first sense.

On either view, the trick will be to justify the claim that the scoring rule have certain formal features X. Then one appeals to a formal argument that shows that for every incoherent credence c, there’s a coherent credence d which is more accurate (by the lights of the scoring rule) than c no matter what the actual truth values are—supposing only that the scoring rule has feature X. Being “accuracy dominated” in this way is supposed to be an epistemic flaw (at least a pro tanto one). [I’m going to leave discussion of how *that* goes for another time]

Ok. But how are we going to justify features of scoring, other than the minimal constraints above? Well, Joyce (1998) proceeds by drawing out what he regards as unpleasant consequences of denying a series of formal constraints on the scoring rule. Though it’s not *immediately obvious* that to be a “measure of accuracy” scoring rules need to do more than satisfy *minimal*, you may be convinced by the cases that Joyce makes. But what *kind* of case does he make? One thought is that it’s a kind of conceptual analysis. We have the notion of accuracy, and when we think carefully through what can happen if a measure doesn’t have feature X, we see that whatever its other merits, it wouldn’t be a decent way to measure anything deserving the name *accuracy*.

Whether or not Joyce’s considerations are meant to be taken this way (I rather suspect not), it’s at least a very clean project to engage in. Take scoring rules to be preferences. Then a set of preferences that didn’t have the formal features just wouldn’t be preferences solely about accuracy—as was the original intention. Or take an objective betterness ordering. If it’s evaluating credence/world pairs on grounds of accuracy, again (if the conceptual analysis of accuracy was successful) it better have the features X, otherwise it’s just not going to deserve the name.

But maybe we can’t get all the features we need through something like conceptual analysis. One of Joyce’s features—convexity—seems to be something like a principle of epistemic conservativism (that’s the way he has recently presented it). It doesn’t seem that people would be conceptually confused if they took their alethic preferences didn’t violate this principle. Where would this leave us?

If we’re thinking of the scoring rule as an objective betterness relation, then there seems plenty of room for thinking that the *real facts* about accuracy encode convexity, even if one can coherently doubt that this is so (ok, so I’m setting aside open-question arguments here, but I was never terribly impressed by them). And conceptual analysis is not the only route to justifying claims that the one true scoring rule has such a feature. Here’s one alternative. It turns out that a certain scoring rule—the Brier score—meets all Joyce’s conditions and more besides. And it’s a very simple, very well behaved scoring rule, that generalizes very nicely in all sorts of ways (Joyce (2009) talks about quite a few nice features of it in the section “homage to the Brier score”). It’s not crazy to think that, among parties agreed that there is some “objective accuracy” scoring rule out there to be described, considerations of simplicity, unity, integration and other holistic merits might support the view that the One True measure of (in)accuracy is given by the Brier score.

But this won’t sound terribly good if you think that scoring rules describe individual preferences, rather than an objective feature that norms those preferences. Why should theoretical unification and whatnot give us information about the idiosyncracies of what people happen to prefer? If we give up on the line that it’s just conceptually impossible for there to be “alethic preferences” that fail to satisfy conditions X, then why can’t someone—call him Tommy—just happen to have X-violating alethic preferences? Tommy’s “scoring rule” then just can’t be used in a vindication of probabilism. I don’t see how the kind of holistic considerations just mentioned can be made relevant.

But maybe we could do something with this (inspired by some discussion in Gibbard (2008), though in a very different setting). Perhaps alethic preferences only need to satisfy the minimal constraints above, to deserve the name. But even if its *possible* to have alethic preferences with all sorts of formal properties, it might be unwise to do so. Maybe things go epistemically badly, e.g. if they’re not appropriately conservative because of their scoring rule (for an illustration, perhaps the scoring rule is just the linear one: s(x,y) is the absolute difference of x and y. This scoring rule motivates extremeism in credences: when c(p)>0.5, you minimize expected inaccuracy by moving your credence to 1. But someone who does that doesn’t seem to be functioning very well, epistemically speaking). Maybe things go prudentially badly, unless their alethic values have a certain form. So, without arguing that it’s analytic of “alethic preference”, we provide arguments that the wise will have alethic preferences that meet conditions X.

If so, it looks to me like we’ve got an indirect route to probabilism. People with sensible alethic preferences will be subject to the Joycean argument—they’ll be epistemically irrational if they don’t conform to the axioms of proability. And while people with unwise alethic preferences aren’t irrational in failing to be probabilists, they’re in a bad situation anyway, and (prudentially or epistemically) you don’t want to be one of them.It’s not that we have a prudential justification of probabilism. It’s that there are (perhaps prudential) reasons to be the kind of person such that its then epistemically irrational to fail to be a probabilist.

Though on this strategy, prudential/pragmatic considerations are coming into play, they’re not obviously as problematic as in e.g. traditional formulations of Dutch book arguments. For there, the thought was that if you fail to be a probabilist, you’re guaranteed to lose money. So, if you like money, be a probabilist! Here the justification is of the form: your view about the value of truth and accuracy is such-and-such. But you’d be failing to live up to your own preferences unless you are a probabilist. And it’s at a “second order” level, where we explain why it’s sensible to value truth and accuracy in the kind of way that enables the argument to go through, that we appeal to prudential considerations.

Having said all that, I still feel that the case is cleanest for someone thinking of the scoring argument as based on objective betterness. Moreover, there’s a final kind of consideration that can be put forward there, which I can’t see how to replicate on the preference-based version. It turns on what we’re trying to provide in giving a “justification of probabilism”. Is the audience one  of sympathetic folk, already willing to grant that violations of probability axioms are pro tanto bad, and simply wanting it explained why this is the case (NB: the pragmatic nature of the Dutch Book argument makes it as unsatisfying for such folk as it is for anyone else). Or is the audience one of hostile people, with their own favoured non-probabilistic norms (maybe people who believe in Dempster-Shafer theory of evidence)? Or the audience people who are suitably agnostic, initially?

This makes quite a big difference. For suppose the task was to explain to the sympathetic folk what grounds the normativity of the probability axioms. Then we can take as a starting point, that one (pro tanto) ought not to violate the probability axioms. We can show how objective betterness, if it has the right form, would explain this. We can show that an elegant scoring rule like the Brier score would have the right form, and so provide the explanation. And absent competitors, it looks like we’ve got all the ingrediants for a decent inference-to-the-best-explanation for the Brier Score seen as the best candidate for measuring objective (in)accuracy.

Of course, this would cut very little ice with the hostile crowd, who’d be more inclined to tollens away from the Brier score. But even they should appreciate the virtues of being presented with a package deal, with probabilism plus an accuracy/Brier based explanation of what kind of normative force the probability axioms have. If this genuinely enhances the theoretical appeal of probabilism (which I think it does) then the hostile crowd should feel a certain pressure to try to replicate the success—if only to try to win over the neutral.

Of course, the sense in which we have a “justification” of probabilism is very much less than if we could do all the work of underpinning a dominance argument by conceptual analysis, or even pointing to holistic virtues of the needed features. It’s more on the lines of explaining the probabilist point of view, than persuading others to adopt it. But that’s far from nothing.

And even if we only get this, we’ve got all we need for other projects  in which I, at least, am interested. For if, studying the classical case, we can justify Brier as a measure of objective accuracy, then when we turn to generalizations of classicism—non-classical semantics of the kind I’m talking about in the paper—then we run dominance arguments that presuppose the Brier measure of inaccuracy, to argue for analogues of probabilism in the non-classical setting. And I’d be happy if the net result of that paper was the conditional: to the extent that we should be probabilists in the classical setting, we should be analogue-probabilists (in the sense I spell out in the paper) in the non-classical setting. So the modest project isn’t mere self-congratulation on the part of probabilists—it arguably commits them to a range of non-obvious generalizations of probabilism in which plenty of people should be interested.

Of course, if a stronger, more suasive case for the features X can be made, so much the better!

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Subject-relative safety and nested safety.

The paper I was going to post took off from very interesting recent work by John Hawthorne and Maria Lasonen that creates trouble from the interaction of safety constraints and a plausible looking principle about chance and close possibilities. The major moving part is a principle that tells you (roughly) that whenever a proposition is high-chance at w,t, then some world compatible with the proposition is  a member of the “safety set” relevant to any subject’s knowledge at w,t (the HCCP principle).

It’s definitely worth checking out Williamson’s reply to H&L. There’s lots of good stuff in it. Two relevant considerations: he formulates a version of safety in the paper that is subject-relativized (one of the “outs” in the argument that H&L identify, and defends this against the criticisms they offer). And he rejects the HCCP principle. The basic idea is this: take some high-but-not-1-chance proposition that’s intuitively known e.g. the ball is about to hit the floor. And consider a world in which this scenario is duplicated many times—enough so that the generalization “some ball fails to hit the floor” is high-chance (though false). Each individual conjunct seems epistemically on a par with the original. But by HCCP, there’s some failure-to-hit world in the safety set, which means at least one of the conjuncts is unsafe and so not known.

Rejecting HCCP is certainly sufficient to get around the argument as stated. But H&L explicitly mention subject-relativization of safety sets as a different kind of response, *compatible* with retaining HCCP. The idea I take it is that if safety sets (at a given time) can vary,  *different* “some ball hitting floor” possibilities could be added to the different safety sets, satisfying HCCP but not necessarily destroying any of the distributed knowledge claims.

I see the formal idea, which is kind of neat. The trouble I have with this is that I’ve got very little grip at all as to *how* subject-relativization would get us out of the H&L trouble. How can particular facts about subjects change what’s in the safety set?

I’m going to assume the safety set (for a subject, at a given time and place) is always a Lewisian similarity sphere—that is, for some formal similarity ordering of worlds, the safety sphere is closed downwards under “similarity to actuality”.  I’ll also assume that *similarity* isn’t subject-relative, though for all I’ll say it could vary e.g. with time. The assumptions are met by Lewis’s accout of counterfactual similarity—in fact, for him similarity isn’t time-relative either—but many other theories can also agree with this.

The assumption that the safety set is always a similarity sphere (in the minimal sense) seems a pretty reasonable requirement, if we’re to justify the gloss of a safety set as a set of the “sufficiently close worlds”.

But just given the minimal gloss, we can get some strong results: in particular, that safety sets for different subjects at a single time will be nested in one another (think of them as “spheres around actuality”–given minimal formal constraints, Lewis articulates, the “spheres” are nested, as the name suggests).

Suppose we have n subjects in an H&L putative “distributed knowledge” case as described earlier. Now take the minimal safety set M among those n subjects. This exists and is a subset of the safety sets of all the others, by nesting. And by HCCP, it has to include a failure-to-hit possibility within it. Say the possibility that’s included in M features ball k failing to hit. But this means that that possibility is also in the safety set relevant to the kth person’s belief that their ball *will* hit the ground, and so their actual belief is unsafe and can’t count as knowledge—exactly the situation that relativizing to subjects was supposed to save us from!

The trouble is, the sort of rescue of distributed knowledge sketched earlier relies on the thought that safety sets for subjects at a time might be “petal shaped”—overlapping, but not nested in one another. But thinking of them as “similarity spheres”, where similarity is not subject relative, simply doesn’t allow this.

Now, this doesn’t close off this line of inquiry. Perhaps we *should* make similarity itself relative to subjects or locations (if so, then we definitely can’t use Lewis’s “Time’s arrow” sense of similarity). Or maybe we could relax the formal restrictions on similarity that allow us to derive nesting (If worlds can be incomparable in terms of closeness to actuality, we get failures of nesting—weakening Lewis’s formal assumptions in this way weakens the associated logic of counterfactuals to Pollock’s SS). But I do think that it’s interesting that the kind of subject-relativity of closeness that might be motivated by e.g. interest-relative invariantism about knowledge (the idea that how “close” the worlds to be in the safety set  depends on the interests etc of the knower) simply don’t do enough to get us out of the H&L worries.  We need a much more thorough-going relativization if we’re going to make progress here.

Utility of posting papers in public

I was about to post up a draft of a new paper. And then I picked up on a rather nasty flaw in the argument. So that paper’s now under reconstruction again—until I find a way to patch the gap.

It’s a bit cold-shivery to have almost posted things in a very public way with a major quantifier-shift fallacy right in the centre of them. But I take at least this out of the experience: posting things on blogs is a *very* good way of disciplining yourself on content. At least for me, I’m shifted from a mode where I’m wanting things to work out/patch errors etc—in effect, working on the content of the paper itself—to a mode where I’m looking at it with an eye to potential readers. And it’s the second mode whereby I find I get enough critical distance to reliably spot things that need fixing (be they typos or real errors I’ve missed).

And of course, this is even before the very clever people out there pitch in to helpfully point out all the ways in which things need tightening up or amending. So hooray for academic blogs. But boo to quantifier shift fallacies.

Gradational accuracy; Degree supervaluational logic

In lieu of new blogposts, I thought I’d post up drafts of two papers I’m working on. They’re both in fairly early stages (in particular, the structure of each needs quite a bit of sorting out. But as they’re fairly techy, I think I’d really benefit from any trouble-shooting people were willing to do!

The first is “Degree supervaluational logic“. This is the kind of treatment of indeterminacy that Edgington has long argued for, and it also features in work from the 70’s by Lewis and Kamp. Weirdly, it isn’t that common, though I think there’s a lot going for it. But it’s arguably implicit in a lot of people’s thinking about supervaluationism. Plenty of people like the idea that the “proportion of sharpenings on which a sentence is true” tells us something pretty important about that sentence—maybe even serving to fix what degree of belief we should have in it. If proportions of sharpenings play this kind of “expert function” role for you, then you’re already a degree-supervaluationist in the sense I’m concerned with, whether or not you want to talk explicitly about “degrees of truth”.

One thing I haven’t seen done is to look systematically at its logic. Now, if we look at a determinacy-operator free object language, the headline news is that everything is classical—and that’s pretty robust under a number of ways of defining “validity”. But it’s familiar from standard supervaluationism that things can become tricky when we throw in determinacy operators. So I look at what happens when we add in things like “it is determinate to degree 0.5 that…” into our object-language. What happens now depends *very much* on how validity is defined. I think there’s a lot to be said for “degree of truth preservation” validity—i.e. the conclusion has to be at least as true as the premises. This is classical in the determinacy-free language. And its “supraclassical” even when those operators are present—every classically valid argument is still valid. But in terms of metarules, all hell breaks loose. We get failures of conjunction introduction, for example; and of structural rules such as Cut. Despite this, I think there’s a good deal to be said for the package.

The second paper “Gradational accuracy and non-classical semantics”  is on Joyce’s work on scoring functions. I look at what happens to his 1998 argument for probabilism, when we’ve got non-classical truth-value assignments in play. From what I can see, his argument generalizes very nicely. For each kind of truth-value assignment, we can characterize a set of “coherent” credences, and show that for any incoherent credence there is a single coherent credence which is more accurate than it, no matter what the truth-values turn out to be.

In certain cases, we can relate this to kinds of “belief functions” that are familiar. For example, the class of supervaluationally coherent credences I think can be shown to be Dempster-Shafer belief functions—at least if you define supervaluational “truth values” as I do in the paper.

As I mentioned, there are certainly some loose ends in this work—be really grateful for any thoughts! I’m going to be presenting something from the degree supervaluational paper at the AAP in July, and also on the agenda is to write up some ideas about the metaphysics of radical interpretation (as a kind of fictionalism about semantics) for the Fictionalism conference in Manchester this September.

[Update: I’ve added an extra section to the gradational accuracy paper, just showing that “coherent credences” for the various kinds of truth-value assignments I discuss satisfy the generalizations of classical probability theory suggested in Brian Weatherson’s 2003 NDJFL paper. The one exception is supervaluationism, where only a weakened version of the final axiom is satisfied—but in that case, we can show that the coherent credences must be Dempster-Shafer functions. So I think that gives us a pretty good handle on the behaviour of non-accuracy-dominated credences for the non-classical case.]

[Update 2: I’ve tightened up some of the initial material on non-classical semantics, and added something on intuitionism, which the generalization seems to cover quite nicely. I’m still thinking that kicking off the whole thing with lists of non-classical semantics ain’t the most digestable/helpful way of presenting the material, but at the moment I just want to make sure that the formal material works.]