The way an anthropologist apprehends the ground, tastes things and feels them, has an impact both on the data he collects and on the analyses he then produces. Psychotropics, subtitled The ayahuasca fever in Amazon forest, illustrates this epistemological evidence. This book by Jean-Loup Amselle is indeed a particular book from the anthropological point of view, since, in the name of rationalism to defend, he criticises social practice (the “vegetalism”), by issuing partial and partial truths (Clifford 1986), which are politically positioned, which significantly harms the understanding of the phenomenon investigated. As social science researchers increasingly value the inductive “rooted” theory (Glazer & Strauss 1967) and in which the facts must come out of the field, the author proposes a decisive approach, where his assumptions (the “retro-voltage” and his verdict (the emergence of “a new religion”) are posed before the investigation is conducted, and where the data that interest him is selected and inserted in a preconceived mould.