Vagueness, Ambiguity and Perceptual Bistability

1. Introduction. While vagueness and ambiguity have been opposed in the semantic domain (vagueness is often described as meaning underdetermination, ambiguity as meaning overdetermi-nation, see Fine 1975; moreover, vagueness is fundamentally a property of concepts, ambiguity a property of stimuli or expressions), Raffman has made the suggestion that within soritical series, borderline cases typically pattern as ambiguous stimuli (Raffman 1994). More precisely, Raffman has put forward the idea that for borderline cases, the instability of our judgements may be akin to the kind of instability we experience in the perception of bistable figures (like the Necker cube, or Jastrow's duck-rabbit). For instance, Raffman considers a series of color shades making a smooth transition from red to orange and notes that " borderline cases are cases for which looking red and looking orange are very much alike ". One observation she makes is that while the color's quality appears to change as the judgment flips, there is also a more basic sense in which the stimulus remains constant. Her conclusion is that " such an effect may amount to a kind of Gestalt switch: there is a similar respect in which (for example) the duck-rabbit " looks the same " while yet " looking different " as it species fluctuates " (Raffman 1994: 53). Though highly suggestive, Raffman's comparison between the kind of instability experienced in vagueness and the kind of instability experienced with bistable figures has not been an object of close study. In the first part of this paper, I propose to tighten up the connection between vagueness and bistability by undertaking a comparison between sorites series and what I call " Fisher series " , namely series of ambiguous figures gradually altered. In the second part, I discuss two consequences of this analogy. The first concerns the ontology of vagueness and the potential inadequacy of epistemicist accounts (see Williamson 1994); the second concerns the nature of the uncertainty characteristic of vagueness.