"the law of Christians which propounds the superiority of monastic life over marriage is false and can’t come from God." However, he also rejected polygamy because "the law of creation orders one man to marry one woman." Yacob became the teacher of Habtu's two sons, and at the request of his patron's son Walda Heywat, Yacob wrote his famous 1667 treatise investigating the light of reason. Little is known of Yacob's later life. However, it is believed that he lived a fulfilled family life in Emfraz, and remained there for the next 25 years. He died there in 1692. Yacob's year of death was recorded by Walda Heywat in an annotation to the Treatise. Authorship controversy The authorship of the Hatata was challenged by Carlo Conti Rossini in 1920, who claimed it was forged by father Giusto d'Urbino, an Italian scholar who worked in Ethiopia. The arguments are extrinsic, based on the manuscripts' recent age, his knowledge of Ethiopic language and culture, the information on Islam also being known by d'Urbino, and the fact that he discovered the two extant manuscripts. In 1934, Eugen Mittwoch put forward linguistic arguments for the inauthentic nature of the Hatata, and scholarly interest in the work waned. Amsalu Aklilu and Ato Alemayyehu Moges argued for the authenticity of the work, based on its nonreligious contents, sentence structure, and the particularity of the Ge'ez used. Claude Sumner wrote in favor of the Inquiry's authenticity in 1976 with statistical evidence showing the duality of authors in their differing Biblical quotations, using five newly found letters of d'Urbino in Rome. Sumner also argued that his knowledge of Ge'ez was worse than originally presented, and that he did not share the ideas of the Hatata at the time he was supposed to have written it. Philosophical work Yacob is most noted for this ethical philosophy surrounding the principle of harmony. He proposed that an action's morality is decided by whether it advances or degrades overall harmony in the world. While he did believe in a deity, whom he referred to as God, he rejected any set of particular religious beliefs. Rather than deriving beliefs from any organized religion, Yacob sought the truth in observing the natural world. In Hatata, Yacob applied the idea of a first cause to produce a proof for the existence of God, thus proposing a cosmological argument in chapter 3 of Hatata: "If I