 ([Certainty] 100.18.) In other words they hold by reason of essence. Also in a discussion of demonstration in [Attainment] 51.15–52.17 al-Fārābī tells us that demonstration must proceed from some primary concepts, and the concepts in question are those that are answers to the questions “what”, “by what”, “how”, “from what”, and “for what”. These are not exactly the questions listed in  Section 4.2 above,  but they are clearly in that territory. Again they indicate that a demonstration should follow from the constitutive properties of the subjects under discussion, i.e., from their essences. 