In Outside Color: Perceptual Science and the Puzzle of Color in Philosophy, Mazviita Chirimuuta engages with the fundamental questions about the metaphysics of colour—whether colours are real and whether they are mind-independent or mind-dependent properties. By engaging in contemporary perceptual science, neuroscience, and vision science, Chirimuuta provides a novel theory of the metaphysics of colour which moves beyond the traditional confines of the debate. This theory—colour adverbialism—presents colours as properties of interactions between perceivers and their environments.

In Chapter 1, Chirimuuta reconstructs the philosophical debate by explicating the problem of colour, placing the debate within its historical context, and highlighting the common assumptions. The philosophy of colour has been primarily concerned with the ontological nature of colours, and how they compare to other natural properties. Part and parcel of this question is where colours are located: are they properties of objects in the extradermal world, or properties of the intradermal world, located exclusively in the mind? Answering this question has proven to be particularly challenging, as colours seem to occupy both the subjective, inner world of sensations and the objective, outer world of physical facts. Chirimuuta surveys the broader historical scope of philosophy of colour in Chapter 2, tracing the shift from Aristotelian theories of perception to the Scholastic realism of Aquinas, to the anti-realism of Galileo and Newton. What this historical archeology unearths is two primary assumptions: colours can be analyzed separately from other visual properties (e.g., shape and depth) and biological vision is on the same epistemic level as machine-aided vision. Additionally, these debates construe colours as belonging to one side of a binary or another (i.e., colours are either primary or secondary qualities, mind-independent or mind-dependent, and either real or illusory).

In Chapter 3, Chirimuuta explores how these assumptions have shaped contemporary philosophy of colour. Specifically, Chirimuuta explores three views: realism, anti-realism, and relationism. Proponents of both realism and anti-realism argue that colours must belong either to the extradermal world of physical facts or the intradermal realm of the mind. However, as Chirimuuta argues, there is a crucial sense in which colours are ‘Janus-faced’—that they are comprised of both inner and outer features—and accordingly, focusing solely on explanations from one side is detrimental to a full explanation of colour. For example, the anti-realist seeking to replace chromatic language (i.e., qualitative terms like blue or red) with neural language (i.e., physical terms like the firing of post-receptor neurons) eliminates the possibility of important scientific research topics, such as the relationship between colors and material properties. The Janus-faced nature of colours becomes more developed in Chapter 4, where Chirimuuta draws on perceptual science, neuroscience, and vision science to demonstrate the relational nature of colour vision. For example, research on segmentation (i.e., the ability to distinguish colour from shape) suggests that there is a reciprocal relationship between chromatic and non-chromatic properties (i.e., shape and/or depth influence colour and vice versa). It becomes clear that neither realism nor anti-realism with their emphasis on a strict inner/outer divide is compatible with this portion of the scientific literature. While relationism emphasizes the relationship between perceiver and environment, no account has been able to sufficiently account for the Janus-faced nature of colours. In the remaining chapters, Chirimuuta takes on this challenge.

In chapter 5, Chirimuuta abandons the key assumption that whether colours are real depends on an isomorphic correspondence between perceptual states and the mind-independent world. She replaces this assumption with a pragmatic view of perceptual systems compatible with the aforementioned scientific literature on perception. In chapter 6, Chirimuuta develops her view of colour adverbialism, which states that colours are best understood as qualifiers of perceptual processes between perceivers and their environments. Her position breaks with traditional relationism in that colours are linked to both stimuli and perceivers, and takes colour vision as a process which connects perceivers to objects in the extradermal world, thus offering a fresh position compatible with the Janus-faced nature of colours and the scientific literature on perception.

In Chapter 7, Chirimuuta addresses the objection that colour adverbialism fails to distinguish between veridical and non-veridical perceptions of colour. Chirimuuta argues that, while there are some cases of genuine misperception, these cases should not be analyzed in terms of ascribing the wrong colour to a given object, but instead as not seeing things in the way in which we are accustomed, under certain circumstances. Chirimuuta’s response to each type of misperception is particularly subtle, and there is much to benefit readers from her discussion here. In Chapter 8, Chirimuuta moves on to tackle perhaps the biggest objection facing colour adverbialism: that our experience does not present colours as relational, but as nonrelational properties of objects in the extradermal world. Chirimuuta breaks this phenomenological argument down and introduces two criteria that proponents of this argument must satisfy. First, phenomenologists must state what it would take for colour phenomenology to be consistent with relationism, and second, they must be able to demonstrate that the nonrelational aspect of colours is seen and not surmised. Chirimuuta demonstrates that neither criterion is satisfied and concludes that, at best, phenomenology is neutral about the relationality of colours.

Chirimuuta’s book is rich, beautifully written, and persuasively argued. It provides a novel theory for the metaphysics of colour, which should be of great interest to philosophers of perception and philosophers of history and science. Theoretically inclined scientists and philosophers of science interested in methodological questions may also find this book of interest, for Chirimuuta’s colour adverbialism might have important implications for the ways in which experiments on perception are designed. While it remains an open question whether colour adverbialism can extend more generally to philosophy of mind, the challenges and thought-provoking ideas are worthy of engagement.