In the Abbasid Baghdad, around the year 800 AD, the Christian chief physician of the Caliph Harun ar-Rashid, BoktiSoʿ, asks his patriarch Timothy I, Catholicos of the Church of the East about the condition of the soul after the death. His answers constitute the second of his letters. It is an original synthesis of the most relevant statements of the Syriac Christians psychological thought between 4th and 8th Century. Timothy describes the human soul life mixing Greek Philosophy and Antiochean Anthropology and at the same time he secularizes the dead body, legitimating the study of the anatomy from a religious point of view. Timothy also defends the interaction between free will and divine providence in the entrance and the exit of the soul from the body. This defense reflects the relevance that the Muslim debate on freedom and determinism had inside the Christian circles of Medicine. The book inscribes Timothy's letter to Rabban BoktiSoʿ in the history of the Syriac anthropological reflection during Late Antiquity. Here translated for the first time in French, Timothy's treaty appears as the culmination of this cultural process.