What is “soma”? In The Tawny One: Soma, Haoma and Ayahuasca, Matthew Clark aims to shed new light on the plant called soma in Sanskrit and haoma in Avestan, whose juice was consumed as a sacrament in Vedic and Zoroastrian ritual. His thesis is bold and refreshing. Soma/haoma, he argues, was not a single plant, but many plants blended as a concoction, with ingredients varying over time and space. Arguing that viable plants for this purpose possessed chemical constituents with psychoactive effects, Clark compares soma/haoma to ayahuasca, a brew of multiple plants used to engender psychedelic experiences throughout the Amazonian region. To be clear, Clark does not claim that soma/haoma was ayahuasca, or even made from the same plant species; instead, he calls it an “ayahuasca analogue”. In this way, he aims to reorient the scholarly debate – which for centuries has sought out a single plant – towards an assessment of a wide array of plants in Central and South Asia, along with the possible effects – physiological, cognitive, and spiritual – that ingesting such substances could produce.

Ever since it attracted the attention of nineteenth-century Orientalists, soma/haoma's botanical identity has remained one of the great unsolved mysteries in the study of religion. Given the sparseness of the archaeological record, our primary sources are linguistic and literary. Derived from an Indo-Iranian verb root, soma and haoma mean “that which is pressed” – the terms literally refer to the process of extracting juice or sap from a plant for ritual use. Yet in Vedic and Avestan texts the words denote not only the juice, but also a drink made from the juice, the plant itself, and the plant-deity Soma/Haoma. Complicating the issue is the fact that our texts lack unambiguous descriptions of the plant's botanical features, method of preparation, and effects; and refer to different kinds of soma/haoma along with numerous substitutes. Based on the evidence at hand, it seems that soma/haoma was a plant with stems, ranging in colour from tawny brown to golden. Practitioners would soak it in water until it swelled, beat it to extract the juice, strain the impurities, and add milk to cut the bitter taste. After imbibing the mixture, they would feel wakeful, inspired, and intoxicated; in Vedic texts, soma is called amṛta (“the immortal”) – drinking it confers immortality.

Scholars have proposed many different plants as the original soma/haoma, including cannabis, ephedra, fly-agaric mushroom, and Syrian rue. Each of these is psychoactive in its own way, with effects varying according to dose and circumstances. This is the intractable challenge of the soma/haoma debate: while identification hinges on the plant's effects on people, these effects remain murky, subjective, and contested. Since R. Gordon Wasson's influential Soma: Divine Mushroom of Immortality (New York, 1968), which made the case for the fly-agaric based on reports of Siberian shamanism, scholarship on soma/haoma has been divided into two camps: those who think the substance had psychedelic effects (“entheogenic” being the current term of art); and those who deny this. In making the case for an ayahuasca analogue, Clark allies himself with the entheogenists.

Although a slim volume, The Tawny One is densely annotated. After introducing the soma/haoma problem and giving a synopsis of his thesis, Clark provides a comprehensive and up-to-date survey of primary sources and previous scholarship (chapters 2–5). Next, a series of chapters (6–12) evaluates arguments on the botanical identification of soma/haoma, taking each of the major plant-candidates in turn. “Altered states of consciousness and demarcation criteria” (chapter 9), which treats mysticism, phenomenology, and cognitive approaches to religious experience, is a welcome interlude, shifting the focus from plants to people.

Clark's main contribution arrives with “Many plants are soma/haoma” (chapter 13), which argues that no single plant is the original soma/haoma; rather, soma and haoma designate a concoction of multiple plants with psychedelic effects. Although at times this would seem to suggest, bewilderingly, that every plant might have been soma/haoma, the “many plants” thesis is an ingenious way to explain the disparate evidence at hand. Clark's obsessive sifting of clues also reminds us how much remains to be discovered about botanical knowledge in the ancient world. In “A renewed case for a psychedelic: ayahuasca” (chapter 14), Clark explains how ayahuasca is prepared from the Banisteriopsis caapi vine, which contains monoamine-oxidase (MAO) inhibitors, and the leaves of the Psychotria viridis, which contains the alkaloid N-N dimethyl-tryptamine (DMT). Consumed together, these constituents engender visions, ecstasy, and a sense of rejuvenation. He contends that a chemical reaction with identical effects could have been achieved using any number of plants in premodern Central and South Asia. One of the best-known MAO inhibitors is Syrian rue (peganum harmala; pictured on the book's cover), while the possible plant-sources of DMT continue to expand as ethnobotanists make new discoveries (including perhaps kuśa/darbha grass, used throughout Vedic ritual). “Vedic and ayahuasca rituals” (chapter 15) compares textual accounts of soma rites with ethnographic accounts of rituals in the Brazilian Santo Daime church, where ayahuasca is the main sacrament; in Clark's estimation, psychedelic experiences constitute the church's raison d’être. Along these lines, he invites us to consider “a different way of looking at ancient Vedic and Zoroastrian ritual”, namely, as religious institutions that “developed primarily as vehicles for an entheogenic trip” (p. 170).

Several chapters on Greek mystery rites (which he suggests may also have utilized an ayahuasca analogue) and “The Bronze Age origins of entheogenic cults” (chapters 16–18), though interesting as a comparative excursus, undermine the integrity of the argument and would have been better relegated to an appendix. The book finishes with potential rejoinders to the ayahuasca proposition and thoughtful concluding remarks (chapters 19–20).

The Tawny One is an interdisciplinary work of comparison, with all the promise and peril this entails: with its wide scope and heavy reliance on scholarship from diverse fields, the book risks rankling many and satisfying few. Indologists may object that Clark does not offer substantially new readings of Sanskrit materials, while South Asianists may deem his foray into ayahuasca irrelevant. But such critiques would miss the intellectual value of Clark's contribution. In much the same way that Wasson productively injected ethnobotany into an ossified philological debate, Clark fruitfully engages perspectives that previous scholarship has discounted: self-experimentation, ritual studies, history of consciousness, and comparative religion. While it may not solve the mystery once and for all, The Tawny One reframes the soma/haoma problem in ways that will greatly benefit future research.