eller’s book sits at the crossroads between philosophy and neuroscience. Keller’s background is in both; he works as a consultant and an adjunct at Columbia University. The book aims to reconsider and open up the discussion about the philosophy of perception through the focus on olfaction rather than the dominant ocularcentric view; what this means for cognition and consciousness; and how a philosophy of olfactory perception might look like. The author invites to review and evaluate “knowledge” and suggests ways to think differently, making the book of interest to geographers and others interested in social space. Traditionally, vision has been the dominant sense considered in the philosophy of perception. Keller’s book takes olfaction as a paradigm or guiding sensory modality instead, transforming the philosophy and understanding of perception into something very different. As Keller argues, a philosophy of olfactory perception would allow, demand, and grant a different language and open up novel ways to study sociospatial dynamics (because of the limitations in using words) and a focus on the nature of perceptual qualities behavior and not their arrangement in space and time. Keller concludes that a feasible and solid approach to examine features shared between all forms of perception lies in a modality-neutral theory of perception.

First, Keller introduces perceptual qualities (such as the redness of tomato, the smell of a rose) as the building blocks of perception. All visual perception involves colors, all olfactory perception smells. Strategies to order the multitude of qualities have always existed. In Chapter 1, he suggests a multidimensional quality space that includes perceptual qualities of all sensorial modalities, arranged by relational similarity (for instance, stronger odors are similar to louder sounds and darker colors). Partial perceptual spaces for tones and colors have already been established; however, an olfactory perceptual space has not been set up, yet. He discusses the aspects of orderliness (i.e. reduction of complexity of perceptual qualities) and size of the olfactory perceptual space (i.e. number of dimensions) to discuss the challenge of arranging olfactory perceptual qualities. Keller cites multidisciplinary literature (neuroscience, philosophy, psychology, psychophysics, etc.) with relevant behavioral experiments to discuss how an exhaustive quality space could be constructed. This is beneficial to shape and to animate perceptual quality spaces in general by qualitative aspects such as the relation between modalities (e.g. a figure showing the hypothetical arrangement of smells and tastes in perceptual space demonstrating ‘Vanilla odor’ and ‘sweet’ as spatially more similar and proximate than ‘sweet’ and ‘salty’ in the modality of gustation (25).

In Chapter 2, the author looks at how spaces differ by genome and species and how to compare quality spaces by different perceivers. He suggests a strategy for third-person access to perceptual qualities to make the science of perception objective. How so? Keller examines that perceptual qualities, perceived by different perceivers, first, have to be transformed into the same coordinate system (= ‘perceptual space registration’ (46-49) before they can, second, be compared with each other in one combined space (measuring the quantified distance between perceptual qualities). A third person can access perceptual qualities without knowing the minds of the other perceivers.

Chapter 3 deals with olfactory objects and theorizes space and olfaction. Keller describes a fragile landscape of olfaction:

“olfactory landmarks are less stable over time” (65), “human olfactory perception is spatially unstructured” (66).

In contrast to vision, the nature of perceptual qualities rather than their spatial arrangement is relevant. Keller’s most relevant argument here is that human olfactory perception does not discriminate spatially; the laboratory experiment on the ability to tell whether an odor is applied to the left or right nostril—which humans fail to accomplish—serves as the basic argument. He discusses potential objections such as odor-guided navigation by humans: While it is certainly possible that humans can navigate based on smell (finding the source of an odor in a room), this has to do with the spatial structure of the stimulus and not the spatial structure of olfaction. Odor molecules contain no spatial information; thus, olfaction is independent from specific olfactory objects (“olfaction is not an object-directed process” (85).

Thus, olfactory perception does not involve objects with properties; the properties are free-floating or object-less instead. This complicates practices of spatial mapping: Particularly in vision and touch, the spatial structure of the percept resembles the spatial structure of the perceived stimulus. This is not possible in the case of olfaction. The background knowledge of the perceiver is relevant: When a perfumer smells the molecule Galaxolide, it is phenomenally present for her; when an average customer smells Galaxolide, laundry detergent or fresh laundry is present; when an individual has never experienced the molecule, only some sort of smelliness is describable. In Chapter 4 on the function of perception, the ideas of collecting correct information and guiding behaviors are discussed. Keller mentions that perceiving smells evolved to guide adaptive behaviors; collecting correct information by itself has no adaptive value. This contrasts vision: Hunting a boar is largely dependent upon position and movement.

In Chapter 5, Keller considers olfaction in relation to language and emotion. The relation between olfaction and language is weak: a smell vocabulary is lacking, the access to words for smells fails. However, the connection between olfaction and emotions is strong: olfactory information is used to evaluate food, locations, other humans. The reason lies in neuroanatomy (processing of emotions and olfaction in the limbic system). Keller underlines that the sense of smell is evaluative and not descriptive: smell is to emotion what sight or hearing is to cognition. In Chapter 6, the author describes how olfactory perception is modulated through cognitive processes and penetration from other perceptual modalities as inputs for olfaction. He discusses that cognitive penetration in olfaction is more flexible than in vision (a steak rather smells different than it looks different when hungry or satiated); however, olfaction is penetrated by perception from other modalities very early on. Furthermore, Keller highlights that vision in crossmodal perception (i.e. perception in one modality influences perception in one or more other modalities) often has a sensory advantage over the other modalities:

“what we see influences what we perceive in the other modalities” (142).

In olfaction, cognitive processes and crossmodal perception modulate this modality more so than the other senses; however, he qualifies that this, in the end, has to be examined per case.

In Chapter 7, Keller adds the aspect of consciousness. He highlights that neuronal processes are mostly not conscious and explains that convincing theories about mechanisms of conscious neuronal processes are lacking. Chapter 8 looks at functions of conscious brain activities in olfaction. The author reminds us that the function of perception is to guide behaviors and that the sensory modalities are differently connected to cognitive processes with the aim to fulfill different behaviors. For humans, olfaction as an evaluative sense helps to guide decision behaviors. Keller approaches the conscious versus non-conscious perception of smells as being led by non-conscious processing based on the complexity of resulting behaviors (“behaviors that are visually guided are usually more complex than those that are odor-guided”, p. 179). He sums up that the aspect if conscious or non-conscious processing of olfactory information is independent from the stimulus but dependent on the number of relevant behavioral options. Again, the interaction of the literature triangulation warrants his proceeding.

Finally, the author makes the point that perception differs between modalities as well as species: visual perception has spatial structure whereas olfactory perception does not; for humans, it is not possible to perceive a spatial arrangement of smells. Instead, olfaction is largely about the nature of perceptual qualities, not the arrangement in space and time. Keller ends with a claim to challenge ocularcentrism in order to reexamine the assumptions about perception in philosopher’s vocabulary.

At times, the book reads like a (collection of) presentation(s) or internal discussion. This makes it challenging to follow. In addition, an elevated use of visual intermediaries (graphs, tables, etc.) and graspable examples or analogies (e.g. the analogy with a city map in explaining perceptual space registration; musk anosmia, D3-deafness, and ageusia as examples of third-person access) would have been instructive.

Nevertheless, I see four ways in which the book is beneficial for geography.

First, “doing geography” in the ways how Keller invites us to perceive would enable and unfold other geographies beyond those that are communicated. Particularly the traditional practice of mapping might be challenged. Since odor molecules and olfactory perception have no spatial information for humans, what merits are there for geographers to bind and keep olfaction to particular scales and spaces on smellmaps, for instance? Further thinking is necessary. Several aspects of olfaction and its perception (e.g. olfaction is more complex; difficult to demarcate from other chemical senses) relate to non-representational theory as one way of approaching the topic in geography. Usually, visual perception is seen as a representation of the physical world; olfactory perception rather emphasizes the behavioral responses to stimuli than their physical characteristics; it characterizes a world that is constantly becoming and changing. However, these comments also lead to the question how “we” (beyond disciplinary boundaries) can be certain to experience and know. Thus, methodological questions how to gain epistemological certainty are observable.

Second, the concept of space has a different orientation and theoretical background than geography’s (multitude of) understanding(s). Chapters 1-3 deal to a large extent with the setup and characterization of space. Interacting space and spatial metaphors with human/non-human sensory perceptions as well as behaviors is very instructive. In the book, space is constructed based on the perception of stimuli; thus, this sort of space would clearly focus on its qualities, individual and collective recognition, and temporal change. Since olfactory perception is not mirroring reality but guiding behavior, this approach could help to examine multi-sensorial affection in understanding space and place. A view on how such a world is becoming and in flux as well as how humans and non-humans produce space through their practices and movements are an opportunity.

Third, Keller posits a post-human geography. This is very helpful since it spans a bridge between how humans “use” olfaction versus animals. The author introduces many insights of sensory characteristics from other living beings. Therewith, he intends to carve out the qualities of a philosophy of olfactory perception. In juxtaposing insights from studies of human and animal perception, he succeeds a better understanding of the specificities of sensory experience overall and how its philosophy is affected.

Fourth, the book reveals operational rigor. This is surely a sign of the disciplinary background in philosophy. His tour de force is, methodologically, a nuanced and well-formed interaction and triangulation of relevant literatures from several hard sciences, implying their typical methods of examining as well as building and affirming knowledge. The author draws from insights from several studies in neurosciences, cognitive sciences, and psychology with behavioral and psychophysical experiments. I think that this way of approaching the topic is beneficial since rigor both from philosophy and neuroscience supports clear statements and suggestions how to proceed. While geographers should be wary of not adopting a sort of positivist enthusiasm towards rigor and methods, increased recognition of operational and methodological accuracy in geography is conducive.

The book is highly recommended for those with an interest in philosophy, space, and olfaction. It is also beneficial for readers with an interest to learn about other disciplines’ methodology and ways of argumentation.