This paper analyzes a classification of different types of demonstration introduced by Alfarabi in his Kitāb al-Burhān. Alfarabi identifies eight combinations of demonstrative syllogisms, grouped in function of the different types of per se relations expressed by their premises and conclusions, where terms are definitionally connected with one another. The list contains a total of thirty-nine moods illustrated by a rich array of examples drawn from various scientific disciplines, including arithmetic, geometry, and natural philosophy. The combinations and moods are discussed extensively by Averroes in the section of his Epitome of the Organon devoted to the Posterior Analytics and in his Quaesita on logic. Alfarabi’s classification also possibly inspired a simplified taxonomical effort in Avicenna’s Kitāb al-Burhān.