The demonstration of the absolute simplicity and real oneness of the Necessary Being, on the one hand, and the presentation of an explanation for the possible beings that are consistent with this oneness are among the most important challenges that Islamic philosophy has to deal with. By restricting the reality of being to the Essence of the Necessary, gnostics have solved this problem successfully; however, philosophers' confirmations of an independent and original existence for possible beings based on the principles of (...) causality is inconsistent with the absolute simplicity of the Necessary. All the efforts of Muslim philosophers for solving this problem were unsuccessful until Mulla Sadra's time. By posing the philosophical principle of 'the Truth in its simplicity contains all things', he solved this riddle and demonstrated the individual oneness of being in the light of a new explanation of the principle of causality. In this way, he paved the way for philosophy to have access to the individual oneness of being. This paper aims to explain Mulla Sadra's significant efforts in this regard. (shrink)