The Logic of Divinatory Reasoning Aafke de Vos Abstract: This thesis is a study of the logic of a specific form of mechanical divination, in which the diviner asks binary questions on the basis of previously gained knowledge. A comparison is made between Mambila spider divination, which is deeply embedded in the (partly illiterate) Mambila community of southern Cameroon, and an experiment with university students that resembles this type of divination. The motivation for this thesis stems from the traditional view that primitive cultures ignore the principle of non-contradiction, as well as from studies that have been conducted about the reasoning of illiterates in reasoning tasks. Those latter studies have shown that unschooled subjects experience difficulties interpreting certain reasoning tasks. Since this thesis focuses on a natural practice, those interpretation problems are avoided. By analyzing Mambila spider divination conceptually and formally, we investigate the logic of divinatory reasoning. Several divinatory sessions are formalized, using Inferential Erotetic Logic. The main result of this thesis is that the limitation to binary questions shapes divinatory logic, such that “no” possibly means “no, unless”. Contradictory answers are often not problematic, neither are they ignored. They are, instead, taken as a sign for the diviner to reformulate his questions or think in a different way.