Mystics from all religions have spoken about a unity in existence. They clearly state that there is really only one being that exists in this Universe, i.e. God. At the same time, they have not denied the multiplicity of creatures that appear to us. These two apparently contradictory statements seem to be apparently contradictory to one another. Metaphysics is a discipline that is concerned with existence. For this reason, they take these mystical utterances seriously and often discuss them in their philosophical investigations from various points of view. The mystics have not attempted to prove their claims using rational proofs. Rather, they claim to have directly witnessed the unity of existence in their mystical visions. As a purely rational discipline, philosophy has made attempts to prove this claim of the mystics and to reconcile any apparent contradictions that might exist in their statements. One of the philosophers who made great progress in this area was Mulla Sadra. Mulla Sadra was of the opinion that it is indeed possible to prove that it is only God that truly exists in the Universe with this contradicting the existence of His creatures. He was of the opinion that this unity manifests itself at two levels of being. At a higher level, there is a multiplicity in unity and, at a lower one, there is a unity in multiplicity. Mulla Sadra proves the higher level of unity using a philosophical principle called the Principle of the Simple Reality. He proves the lower level of unity using the idea that the existence of an effect is a copulative being with respect to its cause. At the same time, Mulla Sadra affirms that both of these levels of existence are levels of one individual being. This paper seeks to explain Mulla Sadra's theory of the unity of being at the level of multiplicity in unity as he described it in his famous philosophical treatise, al-Shawahid al-Rububiyyah.