A compelling thought, no doubt inspired by the idea that vagueness is primarily linguistic, has it that beliefs and desires about vague matters are, in some sense, redundant.
A naive version of this idea might maintain that behaviour consists in precise bodily movements and that any explanation of this behaviour in terms of a person's beliefs and desires about vague matters could be equally or better put by appealing only to that person's beliefs and desires about precise matters.
If you happened to know the precise propositions whose truth a person values, and to know how confident they are about all the precise eventualities, one would be able to determine how that person will behave, assuming they're rational.1 This is the thesis I shall be examining:Practical Irrelevance: The ranking of the available actions for a rational agent supervenes on the attitudes (typically beliefs and desires) the agent has towards precise propositions.[130] [131]
It is completely consistent with this thesis that you have attitudes toward vague propositions. Indeed it is consistent that, for reasons of human psychology or convenience, most practical reasoning is conducted in vague terms. The thesis just states that these attitudes towards the vague are not necessary for the conclusion of this reasoning: the ranking, and thus the action that is most rational for an agent to perform, is determined completely by the attitudes a rational agent has to the precise.
The thought is best explained by examples. It seems clear that someone can care about being rich and consequently go about doing things that they believe will make them rich; this much is certainly consistent with Practical Irrelevance. Someone subscribing to this thesis, however, will insist that what is really going on is that the person in question distributes their desires in such a way that they care about having certain precise amounts of money over others, with a preference towards larger amounts.
This is what ‘caring about being rich' amounts to, or at least supervenes on, and so our desire to be rich is in some sense derivative. A similar example can be run with belief: suppose that I can be described as believing that a certain glass is pretty full, and that consequently my credences concerning the various precise percentages that the glass is filled drop off smoothly. According to the above thesis, the practical significance of this belief is exhausted by the effects it has on my credences about the precise things—when it comes to making decisions it wouldn't matter whether I had the vague belief so long as my credences in the precise remained the same.The idea that attitudes to the vague are epiphenomenal is more of a slogan than a specific thesis, and the different precisifications of it bear no more than a family resemblance. While there is a version of the thesis that I like, I shall be giving reasons to resist the relatively striking Practical Irrelevance principle.
For those who think that vagueness is just a matter of semantic indecision, or some other public language phenomenon, the idea that vagueness is practically irrelevant will seem extremely appealing. Presumably vagueness plays little role in our day-to- day non-verbal decision making: on one view the things we want, believe, and make happen are all precise—it is only when we want to latch on to these things using words that vagueness comes into play. If all propositions are precise the practical irrelevance straightforwardly falls out of standard versions of decision theory, in which the action that is most rational for an agent to perform is determined by the contents of the agent's beliefs and desires, and does not depend at all on the way, or mode of presentation, under which they have these beliefs and desires.[132]
Even those who admit the existence of vague propositions might be attracted to Practical Irrelevance. For example, some versions of expressivism about vagueness described in chapter 8 admit the existence of vague propositions, but treat these as mere constructions out of precise things.
There are also linguistic theorists who want to talk about vague propositions by constructing them out of precise things (e.g. as sets of world-precisification pairs). Again, on these views it is natural to think that the important psychological explanatory work can be done by appealing only to our beliefs and desires about the way the world really is, with the aspects of content that derive from the way we talk playing little or no psychological role.[133]It is natural for this kind of theorist to endorse a particularly strong precisification of the practical irrelevance thesis: if two people have exactly the same (relevant) attitudes towards the precise propositions and they have the same actions available to them, they will act in exactly the same way if they are perfectly rational. In what follows, I will offer some reasons to reject this strong version of the practical irrelevance thesis and thus, indirectly, some further reasons to reject accounts of vagueness that predict that attitudes towards vague matters have no important psychological role.
On the other hand, if you’re an epistemicist who thinks that vagueness is just a matter of ignorance then you might expect Practical Irrelevance to fail in a fairly drastic way. Our beliefs about things we are ignorant about are often extremely relevant to us in practical reasoning. If I’m uncertain whether it will rain then my degree of uncertainty will inform my decision about whether to bring a coat or not; being ignorant of some matter is no indicator of its practical relevance. In particular, ignorance about whether p doesn’t necessarily prevent you from caring whether p: if vagueness were merely a matter of ignorance then there would be no reason why someone couldn’t care intrinsically about the vague.
Thus, another pertinent precisification of the practical irrelevance thesis is the view that it is irrational to care intrinsically about vague matters, which appears to be violated by a purely epistemic account of vagueness.
This principle bears directly on the problem we considered in the last chapter: to what extent do all classical accounts of vagueness collapse into some form of epistemicism? The answer I offer here is that it is not rational to care intrinsically about the vague. This, I think, is the precisification of Practical Irrelevance that holds true, and it is explored in chapter 10.I’ll begin in sections 9.1, 9.2, and 9.3 by evaluating some different ways of understanding the practical irrelevance thesis in the context of a standard decision theory. After that, I’ll reconsider the view that vagueness doesn’t involve one being genuinely uncertain about anything, and argue that uncontroversial facts about preferences alone are enough to ground the judgement that vagueness involves uncertainty.
9.1