 Suppose we have constitutive properties of A and B which between them ensure that every A is a B, and likewise constitutive properties which ensure that every B is a C. Then by Barbara we know that every A is a C; but for demonstrative certainty we want “Every A is a C” to be ensured by the constitutive properties of A and C. Some of al-Fārābī’s syllogisms confirm that this is so, considering different kinds of constitutive  property.[17 ]  In some other cases the constitutive properties fail to yield the required sentences, and al-Fārābī comments that the syllogism gives fact but not cause. Exactly why he chooses the cases that he does is not clear.