The nature of mental entities and the demonstration of their existence eventually became central epistemological issues. Avicenna had admitted the existence of distinct mental entities, but without elaborating much. By the time of Fakhr al-Dīn al-Rāzī and Naṣīr al-Dīn al-Ṭūsī, discussions on mental existence were accorded distinct special section in philosophical treatises. Much later, Mullā Ṣadrā discusses these issues at length, for example, in his al-Asfār al-arbaʿa, but in light of his own ontology. In a much less studied work, al-Masāʾil al-qudsiyya, he presents clear, condensed presentations of three important proofs for the demonstration of the existence of mental entities: a teleological proof, a proof based on the ability to judge (between two mental concepts) and a proof based on the universality of mental concepts. Some of his discussion is far more detailed in al-Masāʾil al-qudsiyyah than in al-Asfār al-arbaʿa. This study of those three demonstrations highlights the fact that Mullā Ṣadrā was more concerned with ontological (as distinct from epistemological) questions than with the nature of the correspondence between mental entities and the external world.