 Why-demonstrations may express why (i) a certain attribute holds of a subject or (ii) both why it holds of that subject and why it exists. In this connection, Avicenna aims to draw a distinction between two different kinds of explanation. The fact that the attribute of having the sum of the internal angles equal to two right angles holds of isosceles triangles because they are triangles is an example of the first kind: the attribute is true of something in virtue of its being true of a per se 1 attribute (in this case its genus) of that thing. The fact that vines are deciduous because they are wide-leaved is an example of the second kind. The distinction is parallel to a distinction proposed for Aristotle between application arguments and subject-attribute demonstrations (McKirahan 1992). Avicenna’s account of per se fits neatly into this scheme, where case (aa) corresponds to why-demonstration of type (ii), and case (ba) corresponds to why-demonstrations of type (i) (Strobino forthcoming).