 The source of Avicenna’s classification is a chapter from his Burhān famously translated and encapsulated by the twelfth-century Toledan philosopher and translator Dominicus Gundissalinus (ca. 1110–1190) in his De divisione philosophiae. In the first part, Avicenna presents a basic classification of the types of hierarchical relations holding between various pairs of sciences, based on the corresponding relations holding between their underlying subjects. At the root of the division is a distinction between sciences that differ because (a) they have two distinct subjects or (b) because they treat one and the same subject in different ways. He articulates in full the primary divisions of scientific domains, offering an analysis and categorization of basic types of relations and dependencies (most notably, parthood and subordination). In the second part, Avicenna develops a complementary classification of the criteria of identity and distinctness for the sciences focusing on the ways in which sciences may have elements in common. In this context, he addresses the question from a different perspective—that of the canonical three elements of an Aristotelian science—including not only the subjects, but also scientific principles and questions (masāʾil), i.e., the conclusions of scientific syllogisms (the theorems of a science).