Soritical Series and Fisher Series

In this paper I propose to reassess the question of whether borderline cases in soritical series can be seen as ambiguous cases between two categories, rather than as cases that instantiate neither. In the philosophical literature on vagueness, the two phenomena of vagueness and ambiguity have been mostly and rightly opposed, but mainly in relation to lexical ambiguity (see Fine 1975, Keefe 2000). Despite this, Raffman (1994) has suggested that borderline cases in soritical series might pattern as bistable stimuli, namely as cases for which one's judgment flickers alternatively between two competing categories. In the first part of this paper I propose to adduce further evidence in favor of Raffman's claim, based on the consideration of an intriguing set of visual stimuli originally designed by psychologist G. Fisher in his work on measuring perceptual ambiguity (Fisher 1967). Basically, Fisher showed that two-way ambiguous figures (such as Jastrow's duck-rabbit) can be biassed so as to gradually favour one percept over the other. In the second part of the paper, I propose to take advantage of a particular analogy one may draw between soritical series and Fisher series in order to make sense of the idea that borderline cases are cases for which contradictory verdicts are permissible (see Wright 1994, Schiffer 2003).