Hallucination: Philosophy and Psychology

1. The Philosophy and Psychology of Hallucination: An Introduction

1 The Philosophy and Psychology of Hallucination: An Introduction Fiona Macpherson Few phenomena have played such a vital role in shaping philosophical theories as hallucination, particularly theories in philosophy of mind, perception, and epistemology . When the ordinary man or woman in the street thinks of hallucination, a drug-fueled bizarre perceptual experience is conventionally what springs to mind. The traditional philosophical conception of hallucination encompasses such experience but is broader. The traditional philosophical conception includes perceptual experiences, identical in nature to experiences that could be had while perceiving the world, save only that they are had while not perceiving.1 Such experiences might be ones that conform to the conventional conception of hallucination. One might, when hallucinating, have an experience of the sort that one would have were pink and green spiders to be crawling over the text that you are reading. However, they might also be perfectly mundane and be just like the visual experience I expect you are having now when reading this page. This mundane form of hallucination is particularly important in philosophy, as philosophers have often contemplated whether all of one’s perceptual experiences to date could have been hallucinatory. Might you be the subject of mass deception, carried out by an evil daemon or by aliens who are artificially stimulating your brain, trapping you in a merely simulated world? In addition, this philosophical conception makes room for experiences that, were the subject to take them at face value, would seem to be perceptual experiences but could never be had when perceiving, at least when perceiving accurately, simply because the world could never be as the experience presents it to be. For example, it might be possible to hallucinate colors that do not and could not exist in the 1. As I explain in more detail in section 1, I use “perceive” as a success verb to indicate perception rather than hallucination, but I use “perceptual experience” to name the kind of state that occurs in both perception and hallucination. Thus note that hallucinations—states not involved in perceiving the world—are nonetheless typically referred to as “perceptual experiences.” I follow that usage in this introduction. 2 F. Macpherson world.2 Or it might be possible to hallucinate a geometrically impossible spatial configuration.3 Although the consequences of the existence of hallucination have been much explored and debated, alternatives to the traditional philosophical conception of hallucination have, until recently, received little attention. However, two emerging strands of research have brought to light other conceptions of hallucination. One of these is scientific evidence about people who actually hallucinate. Evidence from psychology, neuroscience, and psychiatry has shed light on the functional role and physiology of actual hallucinations. The second strand is the development of a philosophical theory of perception known as disjunctivism. Some disjunctivist theories have as part of their ontology a radically new and different conception of hallucination. Once these different conceptions of hallucination are made clear, we can then compare and contrast them. We can ask whether there is or could be any evidence to think that some of them, or all of them, exist or could exist. And we can try to determine whether the traditional debates about the upshot of the existence, or possible existence, of hallucination are transformed by these differing conceptions. Reflection on these different notions of hallucination has the potential to transform many traditional debates in philosophy concerning the nature of the mind, perception , and our knowledge of the world. It has the potential to radically alter our approach to, and answers to, traditional philosophical concerns about knowledge and the mind. In addition, clarifying the different conceptions of hallucination will be of value to scientists when they are trying to determine the nature of hallucination in patients, and to clinical medics who are trying to treat them. The nature of hallucination is therefore of great philosophical, theoretical, and practical importance. These are the issues that the essays in this book engage with. They are written by philosophers of many stripes and by scientists. In this introduction, I aim to achieve a number of goals. I wish to provide an introduction for scientists, philosophers, and other academics who want to understand the philosophical debate about perception 2. See Crane and Piantanida (1983). One might think that such “novel” colors are not just not actual but impossible if one held an objective physicalist view of colors such as that endorsed by Byrne and Hilbert. They claim, “The best description of a world with...