'Boundary-shifting' approaches to the Sorites paradox propose that Soritical reasoning only looks convincing because the boundary for the correct application of a vague predicate shifts as we consider different items in the Sorites series. On every occasion, the predicate has some precise boundary, but whenever we consider two successive elements in the series our very doing so makes it the case that the boundary does not lie between those two items. As Delia Graff puts it in this paper, ‘the boundary can never be where we are looking’. This boundary-shifting effect is taken to be responsible for the prima facie plausibility of the (false) major premises in Sorites arguments.
But there is a limited range of possibly-acceptable positions for the boundary of a vague predicate (at least, this is true for many vague predicates). Graff accepts this - she describes constraints of the kind I have in mind here as ‘clear-case constraints’ - and it is very hard to see how anyone could deny it.
However, familiarly, what counts as a ‘clear case’ for the application of a vague predicate is itself a vague matter. So clear-case constraints seem to determine a range of acceptable positions for the boundary of a vague predicate which range is itself vaguely defined. (Of course, there will also be other constraints on the extension of the predicate on any given occasion, but these are not relevant here.)
If we adopt any kind of boundary-shifting view, then, we should consider that one important kind of vagueness associated with a vague predicate is vagueness in the range, across contexts, of acceptable positions for the boundary. And this seems to raise issues. For one thing, it looks as though the full account of the vagueness of our original predicate ('is red', say) will in the end have to make mention of the vagueness of another predicate ('is a clear case of redness'). And a similar thing will happen when we come to give an account of the vagueness of the latter predicate, and so on ad infinitum.
Two questions about this:
1. Do we ever really get a proper account of the vagueness of the original predicate if this infinite regress gets going?
2. Isn't the first step problematic enough by itself? Can we really hope to make progress in understanding the vagueness of 'is red' with an account that invokes the vagueness of 'is a clear case of redness'? Surely our understanding of the latter derives from our understanding of the former, not vice versa?
I'd be interested to hear comments and/or indications of where to look for discussions of this sort of point in the literature. (I know Graff has a paper forthcoming in the Proceedings of the Aristotelian Society on boundary-shifting and higher-order vagueness. Anything else?)