What has so far been posited in scientific debates or centers concerning the Transcendent Philosophy's critique of ontological issues in Illuminationist Philosophy has mainly consisted of judgmental discussions and criticisms of Suhrawardi's arguments from the viewpoint of Mulla Sadra and his followers. In fact not much has been said concerning the conceptual aspects of this issue. Nevertheless, one can expect Suhrawardi. Who is considered to be one of the first semantic analysts of Islamic philosophy, to have dealt greatly with a conceptual analysis of existence and quiddity and the relation between them. However, Mulla Sadra has a different view in this regard, and in his critiques of this issue, even before entering judgmental and demonstrative discussions, he maintains that there are some scientific defects in the accurate understanding of the concepts of existence, light, and quiddity. Points of conflict between Peripatetic philosophy, that is, those who believe in the objectivity of existence, on the one hand, and the principiality of quiddity in Illuminationist Philosophy, on the other, have arisen as a result. He believes that such defects led Suhrawardi towards the principiality of quiddity.The present paper, while comparing the concepts of quiddity, existence, and light in Illuminationist Philosophy and the Transcendent Philosophy, explains the differences between the principiality of quiddity, principiality of light, and the principiality of existence. Finally, it provides a critique of the ideas of the advocates of Suhrawardi's principiality of light and Mulla Sadra's principiality of existence, as well II as an analysis of some of Suhrawardi's words which have created some ambiguity j concerning the above issue.