In his book Deflating Existential Consequence (which I’ve been discussing in the last several posts), Jody Azzouni argues that ontological commitments of a theory aren’t necessarily signaled by the presence of existential quantifiers when the theory is put into a regimented language. Instead, he thinks there should be an existence predicate that applies only to some posits of the theory. To decide which posits to apply the predicate to, he separates the posits of a theory into thick, thin, and ultra-thin posits, and applies the existence predicate only to the first two (though he suggests that a community other than our own could theoretically have a different practice of making existential commitments – I want to argue that even our own community doesn’t follow his practices).
Thick posits are ones to which we can establish robust, grounded epistemic access, in ways that can be refined, and that allow us to monitor the posit in question. (He goes into much greater detail in the book about each of these four criteria.) Thin posits are ones that we postulate for reasons of theoretical simplicity and explanatory power, subject to the theory continuing to have all the Quinean virtues, and provided there is an explanation for why the posit hasn’t been epistemically detected (if in fact it hasn’t). Ultrathin posits on the other hand are ones that have no requirement on their postulation – characters in fiction are perhaps the paradigmatic example of them. They don’t need to pay their “Quinean rent”, as he calls the requirements on thin posits.
It seems clear to me that we might not want to be committed to ultrathin posits, and also that we should be committed to thick and thin posits. However, it’s less clear that there are any truly ultrathin posits in his sense, or that these three categories exhaust the posits of any theory. Azzouni himself seems to suggest at several points that posits can be thick but not thin (as he says most postulation of individuals is – individuals rarely play any important role in the effectiveness of a theory), thin but not thick (like all the stars outside our light-cone, rabbits that have never been seen by humans, viruses that we haven’t yet detected), or both thick and thin (like the Sun, planets in our solar system, and various other things). But he also suggests that there are posits that are neither thick, thin, nor ultrathin.
It’s a subtle matter whether the theoretical links between what’s implied to exist on the basis of theory and what we’ve forged thick epistemic access to is tight enough to justify the conclusion that what’s theoretically posited is actually thin. Imagine, for example, that a particularly well-attested theory implies the existence of a certain subatomic particle, but that it also follows from that theory that the energies needed to actually forge thick epistemic access to such an object are (forever) vastly beyond our capacities. Physicists, as a community (so I claim), would not commit themselves to the existence of that particle on purely theoretical grounds – no matter how much empirical support in other respects such a theory had. (pg. 147)
Theologians, or some of them, anyway, are perhaps different in this respect. Proofs of God, if any could be found, would amount to the acceptance of a rather unique posit on the basis of purely theoretical virtues. … But theologicans, after all, are a desperate lot. (pg. 148)
However, these examples don’t seem to exhaust the sorts of posits that would appear in this leftover category. If the analogy in the physics case is supposed to be with the top quark (which he discusses in a footnote later on), then my understanding is that physicists in that case thought that they couldn’t even exist without the very high energies, not just that we couldn’t see them. Thus, the top quarks weren’t even a posit of the theory until the accelerator experiments were done – until then they were merely a possibility of the system, not an actual part of it. A better example in this case (and closer to home, for me) is the supposed Super Barn supermarket in Canberra. Many people have independently mentioned it to me – someone suggested I should shop there, someone else used it as a landmark to give directions, and someone else just mentioned it as a major store in the downtown area. Thus, for my best theory of the world, it makes sense to postulate this store, because it would explain the similar discussions of different people who may never even have met one another. However, I have wandered around the downtown area several times, at least once or twice even looking for this store, and haven’t had any epistemic access to it, much less the thick sort Azzouni requires. There is no explanation for why I haven’t seen it, so it seems that he would say that it’s not even a thin posit in my theory. Thus, he suggests that I should say that Super Barn doesn’t exist! This seems to me to be a reductio of his view.
Mark Colyvan makes an objection like this in his review of the book, calling these posits “very thin”. Some of his examples of such entities include dinosaurs and Immanuel Kant (it turns out that the requirements on thick epistemic access are rather stronger than one might expect).
At any rate, I think this is a very fun book to read, even though I disagree pretty strongly with most of the arguments in it (I agree with his nominalism about mathematics, but for quite different reasons).