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Redefining Illusion and Hallucination in Light of New Cases

2016; Wiley; Volume: 26; Issue: 1 Linguagem: Inglês

10.1111/phis.12086

1758-2237

Fiona Macpherson, Clare Batty,

Hallucinations in medical conditions

In this paper, we present new cases of illusion and hallucination that have not heretofore been identified. We argue that such cases show that the traditional accounts of illusion and hallucination are incorrect because they do not identify all of the cases of non-veridical experience that they need to and they elide important differences between cases. In light of this, we present new and exhaustive definitions of illusion and hallucination. First, we explicate the traditional accounts of illusion and hallucination. We then proceed to outline cases of pure property experience—that is, experience as of properties, but not as of objects. We suggest that some might find it to be plausible that olfactory experience is of this kind. We argue that, within instances of such pure property experience, one can identify cases of veridical property perception, illusory property perception and hallucinatory property experience. With these distinctions in hand, we re-examine ordinary cases of experiences as of objects having properties. Drawing on the ideas uncovered by considering pure property experience, we bring to light many new cases of illusion and hallucination within ordinary experience as of objects having properties. These consist in different combinations of veridical perception, illusory perception and hallucination of both objects and properties. In order to accept that these new cases of illusion and hallucination exist in ordinary experience as of objects having properties, nothing turns on accepting the idea that there is pure property experience, or that olfactory experience is an instance of it. Such a conception of experience is simply a tool—a ladder to gain a good vantage point from which one can appreciate that there are these further cases. But this is a ladder that, as Wittgenstein might say, can be thrown away once it is used. Identifying new instances of illusion and hallucination provides much needed, important data for testing theories of experience and perception—theories that are frequently motivated, and should be judged, by their ability to account for cases of illusion and hallucination. Integral to the notions of illusion and hallucination are the notions of worldly objects and properties. Worldly objects and properties exist in physical space, external to, and independently from, the mind of the subject. Such objects and properties will often be external to a person's body, but they need not be, and may include parts and features of a person's body, such as their hand. In this paper, we will suppose that there is a mind-independent reality that includes medium-sized dry goods and the properties that we typically attribute to them—properties such as shape, size, colour, taste and smell.2 We realize that there are metaphysical disputes concerning whether some of these properties are indeed mind-independent. However, nothing that we say will turn on making assumptions about which particular objects and properties are mind-independent. So long as there are some, our points could be made with examples featuring those objects and properties. Veridical perception is contrasted with cases of illusion and hallucination. In veridical perception, one perceives the world, and one perceives it as it is. It is wholly accurate perception. One contrast to this is illusion. The fundamental conception of illusion is that it consists in perception of some form, but it also consists in at least some misperception. That is, it is inaccurate or non-veridical in some respect. Another contrast to veridical perception is hallucination. The fundamental idea of hallucination is that does not amount to perception at all, but it bears some similarity to the mental aspects of perception—either it involves an experience subjectively like, or indistinguishable from, the perceptual experience had in veridical perception, or it involves being in a state that the subject cannot tell apart from such an experience. Any account of illusion and hallucination has to respect these fundamental features of illusion and hallucination.3 However, we take issue with the traditional accounts of illusion and hallucination—accounts that flesh out these fundamental conceptions in a particular way. Trad Illusion: you perceive a (worldly) object but you misperceive one or more of its properties. For example, suppose that, under streetlights, your blue car visually appears to you to be purple. You see the car; the car is indeed there.4 Moreover, suppose that you accurately perceive some of its properties, such as the car's shape and location. Still, you misperceive the colour of the car. Your experience attributes a property to the car that the car does not in fact have. As a result, we say that your experience as of the colour of the car is inaccurate. What you suffer in this case, then, is an illusion with respect to the car's colour. As Smith characterizes it, an illusion is "any perceptual situation in which a physical object is actually perceived, but in which that object perceptually appears other than it really is" (Smith 2002: 23). Cases of illusion exist in modalities other than vision. For example, suppose that there is a cup before you that is at room temperature. You reach out and grasp the cup. But suppose further that, before doing so, you have had your hand in a bucket of ice for some period of time. As a result, when you grasp the cup, the cup feels hotter to you than room temperature. In this case, you feel the cup that exists before you, but you misperceive its temperature. That is to say, you perceive the cup, but your experience is inaccurate with respect to its temperature. What you suffer in this case, then, is an illusion with respect to the cup's temperature. Trad Hallucination: you have an experience as of an object and its properties but there is no (worldly) object, and there are no (worldly) properties, that you perceive in virtue of having that experience. From here on in, we will drop the qualification 'worldly' and leave it as understood. If, at any point, we refer to anything other than worldly objects and worldly properties, we will be explicit about it. The following is an example of hallucination as traditionally conceived: suppose that, although you seem to see a purple car before you, there is no such car. To put it in Smith's terms, although you seem to perceive a car and its properties, you do not actually perceive a car and its properties. What you suffer in this case, then, is a hallucination of the car. As with illusion, cases of hallucination exist in sensory modalities other than vision. For example, you might have an experience as of a feather brushing lightly against your skin. But there might be no feather, or any other object, touching your skin. In that case, according to the traditional definition, you do not actually perceive anything and you suffer a hallucination of a feather. Having set out the traditional notion of hallucination, there are three things to note about Trad Hallucination. First, we must stress the importance of the qualification 'in virtue of having that experience'. This is due to the existence of partial hallucinations. In a partial hallucination, not every object that you experience is hallucinatory. One or more objects are actually perceived, and one or more objects are hallucinated. While philosophers typically consider total hallucinations, partial hallucinations are, as a matter of fact, more prevalent. Consider, for example, after-images. If you stare at a patch of colour and then look at a white wall, for example, you will have an experience as of another patch before you. This patch will appear to be the same shape as the original patch; however, it will appear to have a complementary colour. After-images are commonly taken by philosophers to be cases of hallucination because the patch of colour apparently before you is not there. Despite this, they are not cases of total hallucination. When you experience an after-image, it is not (normally) the case that you do not actually see any objects before you. As you look in front of you, you might see furniture or a rug in the foreground, frames on the wall in your peripheral vision, and so on. Similarly, in the case of clinical hallucinations such as those suffered by Parkinson's patients, subjects may experience hallucinations of loved ones or animals while, at the same time, veridically perceiving the immediate environment in which those things appear to be situated (at least that part of it not occluded by the hallucinated objects). Secondly, Trad Hallucination says nothing about whether a person knows that they are hallucinating or not. Even though it may be more common to portray instances of hallucination in which a person does not realise that they are hallucinating, it is perfectly possible for someone to realise that they are hallucinating—for example, because the content of their hallucination is bizarre, or because they know that they've taken drugs, or because they believe a piece of reliable testimony. Thirdly, it is also important to note that hallucination, even traditionally conceived, cannot be defined as an experience in which, for some hallucinated object, there is no object present with some or all of the properties that you experience the hallucinated object to have. To be sure, it might seem initially tempting to define hallucination in this way. After all, as we have described our example, in which you hallucinate a purple car before you, there is no purple car before you. And many examples in the philosophical literature are described in the same way—that is, as cases in which there is no 'matching' object before a perceiver. But, as Lewis (1980) points out, there could be cases of veridical hallucination. These are cases in which you have a hallucination of an object with certain properties and, by chance, there is such an object in front you. You, however, do not see that matching object. For example, suppose that there is a 'hallucination machine' that, whenever you are hooked up to it, causes you to have an experience as of a certain type of dog, for example a black Miniature Schnauzer. Suppose further that, on one occasion when you are hooked up to the machine, a black Miniature Schnauzer happens to wander in front of you. You don't see that dog, but you are having a hallucination as of such a dog. Given that there is indeed an object before you that matches the experience caused in you by the machine, such a case of hallucination is veridical. Still, it is a case of hallucination nonetheless—just one in which there is in fact an object in the vicinity with the properties that the hallucinated dog appears to have. As a result, hallucination cannot be characterized as an experience in which there is no object present with the properties that you experience a hallucinated object to have. In both Trad Illusion and Trad Hallucination, then, you fall short of accurately perceiving the world. In the illusory case, your experience misattributes one or more properties to a perceived object. In the hallucinatory case, your experience falls short because there is no object and properties that you perceive in virtue of having that experience. Although the traditional definitions make the distinction between veridical perception, illusion and hallucination fairly straightforward, it must be noted that there are some instances of having a perceptual experience that, in practice, are difficult to classify in these terms. For example, it is often difficult to determine, in a particular case, whether another person is suffering from an illusion and thus having an inaccurate experience, or whether they are having an accurate perceptual experience but, despite that accuracy, forming false beliefs about the world.5 Another example of an experience that is difficult to classify occurs when one looks at the Hermann grid (Fig. 1). When one's eyes roam across the grid, one has an experience as of grey patches appearing and disappearing on the white intersections between the black squares. Of course, one realizes quickly that one's experience is not veridical. However, it is not clear what kind of non-veridical experience one is having—illusory or hallucinatory. Is one inaccurately seeing parts of the white lines as having the property of greyness at their intersections—thus undergoing an illusion? Or is one hallucinating grey patches at those intersections, due to the interaction of the grid with one's visual system? It is difficult to decide what the answers to these two questions are and, thus, what the correct description of this case is. Part of the difficulty in answering these questions is that it is not clear what to say in general about certain of our experiences of colour. Do we always experience an object—a patch, say—corresponding to each area that we experience as having a different colour? The difficulty in answering this question accounts for the difficulty in classifying one's experience of the Hermann grid as an illusory or hallucinatory one. If we do always experience an object corresponding to each area of distinct colour that we experience, then the Hermann grid is a case of a hallucination. But, again, it is not clear whether that is the right thing to say in response to this question, so the case may be one of illusion. In what follows, we will not try to answer this question, or the associated question about the Hermann grid. Although there are cases, like the Hermann grid, in which what is experienced is difficult to determine, they have no bearing on whether the traditional distinction between illusion and hallucination is itself clear. They simply expose the fact that it might be difficult to determine under which definition every case falls. Trad Illusion: you perceive an object but you misperceive one or more of its properties. Trad Hallucination: you have an experience as of an object and its properties but there is no object, and there are no properties, that you perceive in virtue of having that experience. We have noted that the traditional definitions are clear; Trad Illusion and Trad Hallucination each capture a set of cases in which there is some failure of perception. Still, as we will argue, the traditional definitions do not cover all failures of perception, and they elide importantly different cases. Before proceeding, we would like to make it clear that, in this paper, we adopt a standard representationalist framework and a causal theory of perception. According to a representationalist account of experience, experiences represent that one's environment is a certain way. How they represent that environment to be is said to be the content of the experience. In representing that the environment is thus and so, experiences represent things like objects, properties, and relations. Nothing turns on our adoption of representationalism.6 For those who think that experiences do not represent, but (at least sometimes) present the environment in a certain way, our talk of experience representing such and such can be read as either experience presenting such and such, or experience being indistinguishable from an experience that represents such and such. According to a causal theory of perception, a sufficient condition for veridical perception is that one has an experience that closely 'matches' the scene around one, and that this experience is caused in the right way by that scene. It is said to be sufficient, and not necessary, in order to recognize the metaphysical possibility of other forms of perception that involve no experience, such as unconscious perception, and the metaphysical possibility of non-causal disjunctive accounts of perception. However, we will treat the condition as both (nomologically) necessary and sufficient for we will not deal with any cases of unconscious perception, and, by adopting the causal framework, we will suppose that all cases of perception in this world conform to the causal model—even though, in other possible worlds, there could be cases of perception that do not conform to it. In the representationalist framework, a matching experience is one in which one's experience has representational content, and that content is correct. One reason for adopting a causal theory of perception is that it is a theory that gives an account of perception in non-perceptual terms. This allows us to usefully employ those other terms to help justify why we think that it is right to classify, as we do, the cases that we will go on to discuss. In other words, it allows us to say something substantive to back up our intuitions about these classifications. If we adopted a disjunctive, naïve realist account of perception, then we would have no further materials with which to work. Such an account just says that one perceives if one bears the perceptual relation to something. Unlike the 'representationalist plus causal theory' of perception, then, this account does not attempt to explain perception in non-perceptual terms. Of course, this is not to say that a naïve disjunctivist could not necessarily agree to the differences in the kinds of cases that we will discuss. Indeed, we think that a test of their theory should be, in part, whether they can handle them. It is just that the representationalist framework provides better resources for explaining those differences. To see that the traditional definitions of illusion and hallucination are not exhaustive, we will consider one account of perception and experience in a modality other than vision—namely, olfaction, and, in particular, orthonasal olfaction.7 The view of the content of olfactory experience that we will consider holds that olfactory experience does not involve the representation of objects. It simply involves the representation of properties; it is a 'pure property' view of olfactory experience. It will not matter for our present purposes whether this is the correct account of the content of olfactory experience—although, given the difference between visual and olfactory experience, it is a plausible prima facie view. All that matters for our purposes is that the pure property view is a possible view of the content of some perceptual experiences. As such, the view is simply a tool employed to reflect more deeply on the nature of illusion and hallucination. Given this, we would like to stress that we will not argue for the pure property view of olfactory experience in this paper and that, in fact, neither of us holds this view of olfactory experience. Macpherson is tempted to hold that in fact olfactory experience includes location and direction information. Batty (2010, 2011) has argued elsewhere that olfactory experience represents that there is something or other 'here', or 'at' the perceiver, that has certain olfactory properties. On the standard view of visual experience, visual experience consists in the representation of objects. In the typical visual experience, objects are represented as having certain properties—shapes, colours, sizes, locations, and so on. For example, we may be aware of a small red car to our left and a large black tractor to our right. There are interesting questions about whether all visual experience involves the awareness of objects having properties, and whether other metaphysical categories, such as relations, can also be represented (as seems likely). There are also interesting questions about whether high-level properties, such as kinds like 'car' and 'tractor', can be represented.8 These are, however, questions that we need not, and will not, answer here. As we indicated above, one view of olfactory experience has it that it does not consist in the representation of objects—neither distal nor proximal objects. The phenomenological differences between visual and olfactory experience motivate such a view. Unlike visual experience, olfactory experience does not seem to represent ordinary distal objects—those objects that we commonly think of as odour sources. For example, you might smell a floral scent and your experience might thereby represent that floral scent. But it is plausible to think that your experience does not also represent the object that has that scent—a bunch of flowers, or a jar of perfume, or any other number of objects. After all, scents can be around when their sources are not, or are no longer, present. What's more, when we do sniff in the vicinity of an odour source, such as brewing coffee, the smell of the coffee does not even seem to occupy more or less determinate locations before us. Rather, the smell is simply present; it pervades, we might say. One might think that this last observation about the phenomenology of olfactory experience supports the view that, in olfactory experience, we don't even represent that proximal objects—objects such as a group of molecules, the air at our nose, a scent, the location at our nose, or even a bare existential quantifier (a 'something')—have certain properties.9 We simply smell that a property is, or properties are, instantiated.10 Although this pure property view of olfactory content is not one that has been defended in the literature, it is certainly one that has been alluded to as the prima facie view of olfactory experience. For example, Chalmers (1996: 8) remarks: "[s]mell has little in the way of apparent structure and often floats free of any apparent object, remaining a primitive presence in our sensory manifold". Chalmers' remarks are seconded by Lycan (1996, 2000) who observes that olfactory experience seems merely qualitative. In particular, Lycan claims that "[p]henomenologically speaking, a smell is just a modification of our consciousness, a qualitative condition or event in us" (2000: 281). Although Lycan goes on to argue, on other grounds, that olfactory experience does in fact represent objects, his quote suggests that the view that olfactory experience represents objects is at odds with the phenomenology of olfactory experience. To be sure, unlike the pure property view that we are considering, Lycan suggests that the properties presented in olfactory experience appear to be properties of the experience itself. Nonetheless, we take these quotes to emphasise the minimal content that one might think that olfactory experiences have. Lycan certainly echoes Chalmers' observations about the lack of apparent structure and, thus, the lack of apparent objects in olfactory experience. To this date, there is no working consensus on what the basic olfactory properties are. As a result, we do not know if this model is correct. Moreover, there is recent criticism of this whole approach in the literature. For example, Wilson and Stevenson (2006) suggest that, behind the difficulties of modelling olfactory quality space, is a false assumption about olfactory experience. This is the assumption that olfactory experience is analytic—that it has various, basic distinguishable components. In contrast to this assumption, they observe that olfactory experience is largely synthetic—that is, that the various properties of the odorant stimulus produce a largely irreducible experience. As a result, Wilson and Stevenson suggest abandoning classification schemes that attempt to model olfactory properties in this way. Although it is important to note these challenges, they need not detain us here. We offer the above simply as an example of a recent empirical view, and of the kinds of properties that might be appealed to by a pure property view of olfactory experience. With that in mind, we will suppose for the sake of convenience, that the above classification is correct and use the properties that it proposes in discussing the pure property view of olfactory experience. Nothing about the controversy over the existence, number, or type of odour primaries challenges the initial observations about olfactory phenomenology that we have made; and it does not challenge the claim that olfactory experience represents properties of some kind. The controversy, then, does not detract from any motivation for the pure property view. The reader can consider the properties that we discuss as placeholders for whatever properties olfactory experience turns out to represent—lesser or greater in number, as the case may be. Employing some of the properties of this empirical view, then, we can say more about the pure property view of olfactory experience. On that view, when one smells, one has various experiences that represent different properties. Some of these experiences might represent a single property, such as floral. Other experiences might represent two or more properties, such as floral, fruity and woody. But properties are all that those experiences represent. an experience in which one perceives fruitiness accurately; an experience in which one perceives fruitiness inaccurately, as (for example) more intensely fruity than it is; an experience in which one perceives nothing, but one has an experience as of fruitiness when there is no fruity smell around. As we will show, once the details of these cases are filled in appropriately, (i), (ii) and (iii) are cases of veridical perception, illusion and hallucination, respectively. Turning to each of the cases, then, (i) is straightforward. It is a case of ordinary veridical perception—albeit the veridical perception of a property. Why should we believe that there can be such a case? One reason is intuition. It seems perfectly possible to conceive of such a case, and especially so in light of the prima facie plausibility of the pure property view for olfactory experience. Another reason stems from examining what is required for veridical perception. As we stated above, we adopt the representationalist framework and the causal theory of perception. Recall also that we have set aside cases of unconscious perception, and have adopted a version of the causal theory according to which a nomologically necessary and sufficient condition for veridical perception is that a perceiver has an experience that 'matches' the object, and that this experience is caused in the right way by that object. In case (i), we have stipulated that the experience matches the property instantiated in one's environment. The experience represents fruitiness and there is indeed just that represented fruitiness in the air. So, case (i) is a case of veridical experience. Of course, not only is matching required for perception, an appropriate causal condition is required as well. At this point, then, it is necessary to say something further about what it is for an experience to be caused in the right way. A promising way of spelling this out is given by Lewis (1980). He takes the case of vision as his example, but one can modify this to cover perception more generally. He claims that one's experience bears appropriate causal relations to the scene experienced if that experience bears a suitable pattern of non-backtracking counterfactual relations to that scene. He explains that what it is for there to be a suitable pattern is that "[t]here is a large class of alternative possible scenes before the subject's eyes, and there are many mutually exclusive and jointly exhaustive subclasses thereof, such that (1) any scene in the large class would cause visual experience closely matching that scene, and (2) any two scenes in different subclasses would cause different visual experience" (1980: 246).13 We stipulate that these conditions hold in (i): there is a large class of alternative possible smelly scenarios before the subject's nose, and there are many mutually exclusive and jointly exhaustive subclasses thereof, such that (1) any smelly scenario in the large class would cause olfactory experience closely matching that scenario, and (2) any two scenarios in different subclasses would cause different olfactory experience. Thus, (i) is a case of veridical perception. Case (ii) is a case of illusion. As we set it out above, case (ii) is one of an experience in which one perceives fruitiness as more intensely fruity than it actually is. Why is it plausible to believe that it is a case of illusion? Let us elaborate on the case. Suppose that there is a fruity odour in the air and it causes you to have an olfactory experience as of a more intense fruitiness than the fruitiness in the air. Your experience represents the property "intense fruitiness", rather than the more accurate "moderate fruitiness". Suppose further that the reason that this happens is because the inside of your nose has been coated with a (non-odorous) chemical that skews the way the receptors in your nose fire in response to odours in the air. In particular, the coating makes all smells seem a little fruitier than they actually are. It is reasonable to think that such a case counts as a case of perception because the Lewisian conditions on perception discussed above are fulfilled. You are having a closely matching experience—albeit not one that is exactly matching, as one is experiencing slightly more fruitiness than is in the air. And we can stipulate that the casual condition is fulfilled: there is a large class of alternative possible smelly scenarios before your nose, and there are many mutually exclusive and jointly exhaustive subclasses thereof, such that (1) any smelly scenario in the large class would cause olfactory experience closely matching that scenario, and (2) any two scenarios in different subclasses would cause different olfactory experience. Thus, we believe that this is a case of illusion; it is an experience in which you misperceive the fruitiness in the air. You do perceive the fruitiness because you are appropriately sensitive to it and other smells (in the sense that you meet the causal condition), but your perception is an instance of misperception because, due to the chemical coating inside your nose, you experience this smell, as you would others