Banisteriopsis caapi is a climbing liana, known in Peru under the name of ayahuasca. According to the pharmacologists, the decoction acts as an emetic and laxative. Like other plants of its family it exhibits monoamine oxidase inhibitor activity. In other words it allows DMT and other hallucinogenic alkaloids to pass through the hemato-encephalic barrier. Yet, Banisteriopsis caapi does play another role during the hallucinogenic trance. Why else would Amerindian and halfcaste populations of western Amazonia name the ingested brewage ayahuasca, the name of this liana ? The native speech informs us that the ayahuasca is una planta maestra, i.e. a teaching plant. Therefore being a purgative and fortifier it is a mediator between the human being and the otherworld. In other words, it contributes to introduce a spiritual principle into the body of the novice shaman. A type of vision originates from this principle. This type of vision aids the shaman in his work.