In the Upper Amazonian vegetalismo traditions of ayahuasca shamanism, shamans drink a highly potent psychoactive brew in order to learn medicine and acquire knowledge from plant teachers. Ayahuasca is thought to provide access to the spirit world, the unseen side of reality. Vegetalistas claim that they are only the intermediaries for spirits that guide and instruct them in the diagnosis and treatment of their patients, and the study of properties of plants and animals. Despite the pan-cultural prevalence of spiritual beings, spirits remain pathologized by the social sciences as “unreal,” the naïve cognitive error or category mistake of non-modernities. In my assessment, it is a critical error to equate “spirits” with autonomous agents transcendent to matter. Rather, I engage in a creative interpretation of Deleuze that approaches spirits as affects, the sub-personal, non-representational singularities of experience that fall outside any distinction of the interiority or exteriority characteristic of subjects and objects. By approaching vegetalismo traditions from a Deleuzian, posthumanist perspective, the centrality of spirits in acquiring knowledge of world and self takes on a radically different valence altogether, no longer a primitive form of proto-science, but a practical form of philosophical askesis that can serve to transform identity, ethics, and is suggestive of what critical theory can undergo as a discipline in learning from the altered states and categories of the “university of the forest.”