In the matter of the Unity of God which is the base of Divine teachings, different views have been presented by the theologians and philosophers of various inclinations and the Sufis and mystics regarding real existence and there is a specific understanding of it in the minds of the public who believe and profess the Unity of God which conforms to the views of the Peripatetic philosophers and theologians. They believe in the plurality of existence and plurality of existents based on the theory of contrast and separation of existents from each other, and believes one of these plural and contrasting existents is God and He is one of many existents, and the cause of all others. Another view believes in a single real and true instance of existence and existents and considers it to be God; however, this view believes that this single existence and existent manifests in the form of many contingents and since the result of this manifestation is the nominal existence of the contingents, their plurality does not damage the aforementioned real unity of existence and existent. In this view, the ignorant from among the Sufis and their elders are in accordance with each other, with the difference that the former limit the existence of God to this level of manifestation as can also be seen in the apparent words of the mystics and this results in the negation of the essence of God; however, the latter group, apart from this level, believe in a level of no condition-no description for His essence but consider that single existence and existent in both levels – be it the level of essence or that of the manifestations – to be necessary; in contrast to the … who, though they believed in the unity of existence, however, they consider existence to be necessary in only level and contingent in the other. In another view, which has been attributed to the transcendental inclination, existence is considered principal and real only in the Necessary Being and is, therefore one; however, in contingent beings, quiddity has been considered as principal and it is clear that quiddities are plural and various. Therefore, in this view, existence is one and is only God; however, existents include both God as well as quiddities and contingents and ultimately, in the views of the great mystics and transcendental philosophers, existence and existents, despite their real plurality, have been described as one; like a person who stands in front of multiple mirrors, in which case, man and also humanity, is plural; but since all those reflections are that same one person, they are one despite their plurality. According to this view, contingent existents are reflections, shadows, and rays of the existence of God in the world and are real and true; in contrast to the view of the Sufi scholars in which contingent existents are nominal. The aforementioned views, except the last, somehow mix polytheism regarding the real existence because in all of them limitations have been considered for God and this necessitates that God is one according to numeric unity; as is indicated by Imam Ali (peace be on him) in the sermon regarding Unity of God at the beginning of Nahjul Balagha: one who has limited Him has numbered Him (Nahjul Balagha, 1980: 40). In this case, God would not be alone and one and other existents – the contingent beings – stand beside Him; whereas lexically, tawhid means unique and singular and God is one in essence, i.e. in existence as well as attributes and actions and contingent beings are rays of His existence just as their attributes and actions are manifestations of His attributes and actions because attributes and actions follow existence and are secondary to it. Therefore, this pure and free-from-all-types-of-polytheism monotheism regarding the real existence must be sought among the monotheistic views of the great mystics and transcendental philosophers and this is Mulla Sadra’s ultimate view and can be concluded from all the discussions in The Transcendent Philosophy of the Four Journeys of the Intellect – even though in the discussion of cause and effect (vol. 2) he has chosen the view of the Sufi scholars (Mulla Sadra, 1981, 2: 292), in another period – probably under the influence of his teacher Mirdamad – he believed the view attributed to the transcendental inclination. This study includes three main topics: The different views regarding Unity of God, the criterion of attaining purity from polytheism, A critical study of the monotheistic views of the philosophers in contrast to the mystics.