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18 A Sense of Reality Katalin Farkas Hallucinations occur in a wide range of organic and psychological disorders, as well as in a small percentage of the normal population (Bentall, 1990). According to usual definitions in psychology and psychiatry, hallucinations are sensory experiences that present things that are not there but are nonetheless accompanied by a powerful sense of reality. As Richard Bentall puts it, “The illusion of reality … is the sine qua non of all hallucinatory experiences” (Bentall, 1990, 82). The aim of this paper is to find out what lends an experience “a sense of reality”: what phenomenological and other features are required for an experience to feel “real,” in the relevant sense? 1 Philosophers’ Hallucinations and Real Hallucinations The notion of hallucination most commonly discussed in philosophy is somewhat different from the notion used in psychology or psychiatry. One source of philosophical interest in hallucinations is a concern with skepticism about the external world. Some familiar skeptical scenarios—that I am deceived by a demon or kept in the Matrix by machines—involve the possibility of a hallucination that is subjectively indistinguishable from a veridical perception. The possibility that we might be in one of these scenarios is claimed to undermine our knowledge of the external world. The “subjective indistinguishability” of hallucinations from perceptions needs further explanation. First, it can mean that a hallucinatory experience presents exactly the same appearance as a veridical perception. If I were now hallucinating because I were in one of the skeptical scenarios, still everything would seem (look, smell, sound, etc.) exactly the same. One commonly used apparatus to further elucidate this concept refers to the phenomenal properties of the experience. The phenomenal properties of an experience are the properties that determine how things feel or seem in an experience : for example, if two experiences both involve feeling cold, they share a phenomenal property; if they both involve something appearing blue, they share another phenomenal property. The hallucinations invoked by the skeptic have exactly the 400 K. Farkas same phenomenal properties as some or another veridical perception. We may say that these hallucinations are “perfect” hallucinations. An alternative understanding of “subjective indistinguishability” requires that the hallucinating subject is unable to tell, just by reflection or introspection, that she is not having a veridical perception. This is a deficiency in the knowledge the subject can activate: she is not in the position to learn (by reflection) that her experience is not veridical. If we accept the previous understanding of hallucination—an experience with the same phenomenal properties as a veridical perception—the inability to introspectively discriminate hallucinations from veridical perceptions follows: one cannot tell them apart, because they appear exactly the same. So sameness of phenomenal properties entails that the experiences cannot be told apart, but the implication doesn’t hold in the other direction: just because a subject cannot activate reflective knowledge that her experience is distinct from another one does not in itself imply that the experiences agree in their phenomenal character.1 There is often a further qualification. There are cases where the subject is unable to activate reflective knowledge that her experience is not veridical because of some general impediment to exercising her reflective capacities, for instance, she is too drunk to do any such thing. Philosophers usually want to exclude such cases, so they require something like an idealized or perfect state of the knower (see, e.g., Martin, 2002; an exception is Fish, 2008). The idea is that one couldn’t discriminate a hallucination from a perception even if one exercised introspection in its best possible form. I have argued elsewhere (Farkas, 2006) that the only way to capture the philosophical notion of hallucination is by appealing to sameness of phenomenal properties; no account of subjective indistinguishability that is formulated merely in terms of the subject’s knowledge will do justice to the fundamental conviction that the world could be very different around me, and yet things could look the same. This isn’t my concern here, but it is one argument for the indispensability of phenomenal properties. Accordingly , throughout this paper, I assume realism about phenomenal properties contra, for example, Daniel Dennett (1991); that is, I assume that there is a fact of the matter 1. The implication in this direction can be questioned because of an independent reason: the possibility of a so-called phenomenal sorites series. Imagine a series of color patches that imperceptibly change from blue to purple; if reflective indiscriminability implied...

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