The issue of the unity of existence is one of the major ontological issues which has come into being in mystic atmospheres and has gradually entered philosophy as well and influenced it to the point that accepting or rejecting it would overshadow all philosophical issues; this is because by accepting the unity of existence, the world takes on a new meaning as causality would depart this world and all creations. Moreover, the unity of existence will provide a different interpretation of Allah Almighty and the religion. Though the philosophers before Mulla Sadra have made arguments in this regard, Mulla Sadra claimed to have made a demonstrative reasoning about it and have defined it exactly and precisely. After Mulla Sadra, the followers of his school of thought also argued about such an issue, the most important of whom is Allameh Tabatabaaii. This paper seeks to present and study the statements of Allameh Tabatabaaii and his argumentations with regard to the unity of personal existence. Allameh Tabatabaaii has proved the unity of existence through the three ways of “copulative existence”, “pure being”, and “infiniteness”.