 Things get more complicated when we turn to  (2).  Following the ancient Greek tradition, Muslim philosophers categorize the foundational principles of the demonstrative sciences into three groups: common notions/axioms (al-uṣūl al-mutaʿārafa), hypotheses (al-uṣūl al-mawḍūʿa), and postulates (muṣādarāt). Roughly speaking, common notions are the most obvious propositions that we can know—the first principles that we grasp. Hypotheses and postulates are not as obvious as axioms. They in principle need to be proved. These two groups of principles are usually distinguished based on the epistemic attitude of the student who is learning them. Hypotheses are the foundational principles that seem plausible to the student even though she has no proof for them. By contrast, postulates seem dubious to the student, in the sense that she might have some feelings and ideas against the plausibility of these principles. The most often repeated example of postulates in the works of medieval Muslim thinkers is probably the parallel postulate of Euclidean geometry. This classification is defended by, among others, al-Nayrīzī (d. 922; in Besthorn &amp; Heiberg 1893: 14–26), al-Fārābi (Al-Manṭiq, chaps. 87–90), Avicenna (al-Burhān, chap. I.12), and al-Ṭūsī (Asās al-ʾiqtibās, chap. V.1.15).