 Avicenna distinguishes between two main types of demonstration in function of the types of predicates involved (in first-figure categorical syllogisms): (a) those in which the middle term expresses a per se 2 attribute of the minor, in which case the major may be either (aa) a per se 2 attribute of the middle or (ab) a per se 1 attribute of the middle; and (b) those in which the middle term expresses a per se 1 attribute of the minor, in which case the major is in most cases (ba) a per se 2 attribute of the middle, and only in two exceptional cases (bb) a per se 1 attribute of the middle (the reason is that usually demonstrations do not aim to prove that per se 1 attributes hold of their subjects, these relations being self-evident and captured by definitions).