The prophetic function that has been known and used during Antiquity and is lost today calls a range of human abilities (perception, intuition, understanding, formulation, anticipation, transmission) relating to the testimonial function. How can the two figures of Prophet and Witness shed light on one another? The religious question of divination and inspiration put aside, the research explores the rhetoric function of the individual carrying a testimony to the world about an event directly experienced, through his own senses. He is vested with the responsibility to deliver this message. The more the event is unexpected for senses, implausible for reason and ineffable for speach, the more the testimonial and prophetic function will be challenged. The upcoming loss of the last witnesses of Nazi extermination camps urges a better understanding of the prophetic / testimonial function. The project focuses on the cognitive, linguistic and rethorical questions of this new challenge for knowledge and society.