It’s a consequence of my favoured version of Everettian quantum mechanics that there is no determinate ontology of macroscopic objects and events. Here’s how I see this working.
At the fundamental level, the ontology of Everettian QM is monistic – there is just one single highly-structured object, the universal state. Call the language in which this claim is true Metalanguage 1. This language trades off minimal ontological commitment for maximal ideological commitment.
By use of decoherence theory to pick out privileged structure from the universal state, we can construct a language in which we can quantify over structures. In this language, Metalanguage 2, there are objects such as branches, branch segments, and so on – all of the entities identified directly with objects on the ‘Literal Fission’ picture I criticize in this post. Because decoherence does not pick out exactly a decomposition basis for the universal state, the translation scheme between Metalanguage 1 and Metalanguage 2 will be vague.
Metalanguage 2 can be thought of as the working language of metaphysical theorizing on this picture. It is flexible enough to be able to refer to everything we want to speak about, whether in or out of the metaphysics room. But it is still a metalanguage; it is unsuitable as an interpretation of our ordinary thought and talk about the macroscopic world. This role is reserved for the object language, which can be explained in terms of Metalanguage 2. My preferred account of the relationship between Metalanguage 2 and the object language is that set out below, in my previous post. According to this view, our ordinary language quantifiers are generally restricted to range over a very special range of entities – fusions of space-time point pairs.
According to this framework, the connection between Metalanguage 1 and Metalanguage 2 is vague, but the connection between Metalanguage 2 and the object language need not be vague. There is still an ineliminable vagueness in ordinary language, but this results from the inexactness of decoherence, rather than from any inadequacy of the translation schema between Metalanguage 2 and the object language. It may be objected that the ineliminable vagueness of the object language is a fault of Everettian QM; but this simply amounts to a complaint that the decoherence-based solution to the measurement problem is faulty. In the absence of any better solution to the measurement problem, we may simply have to bite the bullet of ineliminable non-epistemic vagueness in ordinary thought and talk.
This metaphysical framework is inimical to the epistemicist conception of vagueness. Quantum mechanics allows for no physical facts at the level of Metalanguage 1, not even necessarily unknowable ones, which could determine a precise value for, for example, numbers of branches in Metalanguage 2. If there is no determinate fact of the matter about macroscopic ontology, vagueness in ordinary talk about macroscopic objects and events cannot be a purely epistemic phenomenon. Our concepts cannot have sharp boundaries of application if there are no sharp boundaries in the world for them to latch on to.
If this brand of Everettian QM is even a metaphysical possibility, then it seems to present problems for epistemicism. Presumably epistemicism is intended as an account of vagueness in any language structurally like ours – and if it fails in application to vagueness in Everettian multiverses, do we really have good reason to hold to it even if we’re not ourselves living in such a universe?