Hallucinations certainly are a curse — not only for the people plagued by them, but also for philosophers interested in perceptual knowledge. Hallucinations are mainly non-veridical experiences: They do not match the outside world. But to the subject who has them, they can be indistinguishable from veridical experiences: If I hallucinate voices, this experience might resemble a veridical experience as of listening to people speaking to a stunning degree. So if we want to build our theory of the world based on our experiences, then hallucinations pose a massive problem: Some of our experience are not of the world as it is. The possibility that all our experiences might be hallucinations can lead to skepticism about the external world.

Hallucinations are also a blessing: For philosophers interested in the nature of experience, hallucination are a valuable complementary phenomenon to perception. Both in the case of hallucination and in perception we have sensations, but only in one do they reflect the external world. This suggests that sensory experiences (in contrast to perception) are mainly internally determined, not by our relation to the world. So if we are interested in the minimal conditions of experience in humans, or if we are interested in the connection of experience to other psychological states like action, perception, beliefs and so on, hallucinations are central phenomenon to take into account. But it is this appeal to a common internal factor what Disjunctivism takes issue with. 

Hallucinations may be a curse or a blessing, but they are certainly philosophically and psychologically relevant. The anthology Hallucination: Philosophy and Psychology (MIT Press, 2013), edited by Fiona Macpherson and Dimitris Platchias, sheds light on the psychological and the philosophical double-life of hallucinations with 18 original articles from neuroscientists, psychologists, and philosophers. The collection is based on talks presented at a conference in Crete, so the selection seems mainly determined by attendance. While all papers are illuminating, and although they have been arranged under three main topics, the collection as a whole seems in part like a motley crew: Papers stand beside each other, and reflect different aspects of hallucination, but the interaction is rather low. The book is therefore mainly of interest to specialists. Although Fiona Macpherson’s introduction gives a broad overview, the book as a whole does not serves directly as an introduction to the topic. And it is neither an intensive overview to the topic as a whole, as some forms of hallucination are underrepresented while others are overrepresented: For example, chronoceptive, thermoceptive, or nociceptive hallucinations are not mentioned, yet hallucinating silence is discussed in two papers. Libraries and specialists are the intended audience. This, however, does not diminish the value of this compilation: It reflects the current status of hallucinations, mainly for the discipline of philosophy. 

The book is divided into three parts. The first part consists of five psychological papers. Two papers I found specifically interesting: First, Ksenija Maravic da Silva provides a stunning overview on the risk factors for hallucination: Which circumstances contribute to hallucinating visually later in life? These seem to be mainly dysfunctions of the visual system and those systems that regulate one’s arousal. The last suggests that sleeping problems, specifically troubles staying awake and one’s sleep maintenance over night, might be early signs of proneness to visual hallucinations. Second, Dominic ffytche presents illuminating evidence from neuroimaging studies that hallucinations share an underlying neural signature with illusions and afterimages which differs from that of imagining: Patients suffering from a form of visual hallucinations called Charles-Bonnet-Sydnrome show neural activity during hallucinations comparable to that of actual seeing — its more “the eye’s mind, not the mind’s eye“, as ffytche puts it. Hallucinations, it seems, appear much closer to the failures of perception than to a creative act. This suggests that there is at least an underlying common internal state. One might hope that this factors in to the following discussion on Disjunctivism. Alas, such neural evidence is not discussed further on. The volume would have benefitted from some more interaction between scientific data presented and the philosophical discussion. 

The second part focuses mainly on the philosophical thesis of Disjunctivism with seven papers. Disjunctivism has a prominent position in the philosophy of mind, at the latest since its formulation by John McDowell in Mind and World. Disjunctivism rejects the idea that a veridical perception of a blue vase and a hallucination of that blue vase share a common mental state, e.g. the phenomenal experience of how it feels like to experience a blue vase. According to Disjunctivists, one is in different mental states. In cases of perception, one stands in direct contact with the world: One sees the blue vase out there. In cases of hallucination, one does not. Albeit, it need not be accessible to the subject by introspection whether one is in a state of perception or in a hallucinatory one.

Disjunctivism has been highly popular in academic philosophy. Yet, Disjunctivists have mainly focused on perception and remained suspiciously quiet about their account of hallucination. So the strong focus on Disjunctivism in the volume is certainly justified. Specifically the paper by Susanne Schellenberg on the content of hallucinations was interesting here. She argues for an externalist account: In perception, we relate to external objects. One of the insightful passages is the employment of the extension/intension-distinction usually restricted to linguistic representations. Schellenberg exports this to the realm of sensations: In non-veridical hallucinations, I am then not extensionally aware of an object (because the reference object is not there to relate to), but I am rather intensionally aware: Just like I can be intensionally aware of a green ham-and-egg-sandwich because it is possible to combine these words, I can hallucinate a green ham-and-egg-sandwich because it is possible to combine theses seen properties. If we accept a grounded view, in which my capability to hallucinating certain properties are grounded in my previous encounters of such properties, we get an interesting view of hallucination: It is when we employ perceptual capacities, but fail to single out objects in our environments.

However, the papers on Disjunctivism show in general a lack of interaction with the neural or psychological evidence presented before. The discussion seems to be quite closed to empirical evidence, and none of the papers tries to close this gap. This, I believe, makes for a rather disparate picture: One wonders why the empirical data and the discussions of Disjunctivism are in one and the same book, if they have nothing to say to each other. It would have been nice if this trench between the parts would have been addressed. 

The third and last part of the book focuses on the question of what hallucinations can tell us about the nature of experience. Here, especially the papers by Paul Coates and Katalin Farkas are extremely important: They deal with the feeling of reality our the ability to distinguish between what is real and what is not. These chapters are not only analytically compelling, but also invite to re-evaluate one’s own experiences. They will, I’d predict, prove to be useful inspirations for those being riddled by hallucinations and their caretakers, maybe even inform new methods of self-therapy. This section, as a whole, was a great joy and thrill to read. 

Despite the fact that the parts on Disjunctivism do not integrate clearly with the other chapters, the book is still a tremendous collection of insightful essays. For anybody who professionally works on Disjunctivism, philosophy of perception more generally, phenomenal consciousness, or on the philosophy of psychiatry, this will be an interesting and inspiring book worth one’s look.

 

© 2014 Sascha Benjamin Fink

 

〒       Sascha Benjamin Fink, M. A.

          Institute of Cognitive Science

          University of Osnabrück

          Albrechtstr. 28

          49069 Osnabrück

          Germany

          -&-

〒       Sascha Benjamin Fink, M. A.

          Institut für Philosophie

          Universität Magdeburg

          Zschokkestr. 32

          39104 Magdeburg

          Germany