A Truthsayer is One of an elect group of Reverend Mothers adept in the ways of truthtrance and able to discern and identify falsehood, deceit, and insincerity but also anyone who can detect truth when it is being told. The ability to see truth was also seen as a gift that some individuals were born with. Paul Atreides was identified as one such individual by the RM Gaius Helen Mohiam when she first met him on Caladan in 10191 AG.
The Truthsayers were to represent the spiritual school of the Sisters, acting as their heralds and as the guardians of the Sacred Way to Truth. These holy Reverend Mother Truthsayers were responsible for the restoration and preservation of those qualities, characteristics, and objectives identified with the human cause. The Nine were to investigate the nature of humankind continuously, its customs, practices, societies, passions, weaknesses, and instincts, as well as to assess its cosmic destiny. To realize so monumental a task, each member of the Council of Nine had to have concentrated imagination, audacious conception, savage intention, unbridled psychic sensitivity, and iron will, qualifications then tempered and honed to suit the objectives of the Council.
The special abilities of the Truthsayer supposedly were inherited through Mother Jehanne from the Great Mother (who was reputed to know all things in themselves). Accordingly, only a Truthsayer could properly identify those qualities in one of her Sisters, since the art of divining falsehood was vouchsafed only to a handful of the Elect Reverend Mothers at any one time, led by the Holy Mother, Head of the Cogita Vera Council.
The call to become one of the Council was a heavy burden of responsibility, placing inordinate demands physically, mentally, and spiritually on a woman
Truthsaying was a skill developed and used by the Bene Gesserit. Indeed, the majority of Truthsayers employed during the time of the Faufreluches were Bene Gesserit adepts. According to legend humanity's quest for truth before the Jihad had degenerated into a passion for accumulating and storing facts and data. This obsessive activity was encouraged and implemented by the machine mentality which dominated the times. With machines for testing veracity, men no longer searched within themselves for the eternal verities, the reality behind the illusion that is the phenomenal universe. In reaction to this soul-parching tendency, the Holy Council of Nine was formed as a sacred body of Reverend Mothers whose command of truthtrance imbued them with the power of Truthsayers.
Most eminent among celebrated Truthsayers was the Reverend Mother Mohiam, justly renowned both as practitioner of this art and as historian and analyst of the role of the Truthsayer since its inception just after the Butlerian Jihad. She served as Truthsayer to the Emperor Shaddam IV which offered her a unique, even inspired, vantage point from which to consider the office, function, purpose, and value of her office. The treatise she wrote on the subject was respected, above all, for the authoritative simplicity and elegance of its style and the humility of tone it exhibited in its treatment of so exalted a subject.
Her Prolegomena to the Sacred History of the Council of Nine is deeply indebted to the Sattva Codex, one of the priceless documents committed to the once secret archives of the Bene Gesserit.