A Thirteenth-Century Debate About Avicenna’s Definitions of Differentia

Avicenna (d. 1037) gave two different definitions of differentia, the first, in the Introduction of the Cure, the second, in Pointers and Reminders. In the 1230s, Afḍal al-Dīn al-Khūnajī (d. 1248) was spurred to consider the relative superiority of the two definitions by a question posed fifty years earlier by Fakhr al-Dīn al-Rāzī (d. 1210). Khūnajī argued that the Pointers definition was superior to that in the Cure; his argument turns on criteria used to assess a deduction of the predicables. Toward the end of the thirteenth century, Shams al-Dīn al-Samarqandī (d. 1322) came to the defence of the Cure definition. In his argument against Khūnajī, Samarqandī seems to have relied on claims made in demonstration theory about, first, the hierarchy of the sciences, and second, scientific questions. I conclude by considering whether Rāzī’s theological commitments influence how he approached these logical problems. The material presented in this paper represents a fraction of the contributions made to defining differentia and, in consequence, an even smaller fraction of the discussions of universals in thirteenth-century Arabic logic.