On the basis of the the authenticity of the existence, Mulla-Sadra believes that everything, whether subjective or objective, is real to the extent it exists. But non-existence is nothing and pure untruth. The question that is raised here is whether non-existence has any role in understanding and realization of existence. If yes, then what are those roles? Although existence is the subject matter of philosophy, and it is only existence that has real denotation, our focus on this subject has led us to neglecting of non-existence and its functions. Analyzing the role of non-existence in the emergence and understanding of plurality, and all depend on an understanding of non-existence, its meaning and realizations. Mulla-Sadra considers no authenticity for non-existence and defines it as the lack of existence, but he implicitly considers a role for it in understanding of existence. Addressing the definition of non-existence, its types and its applications in various areas, this paper studies the importance of non-existence in Sadra's philosophy. Accordingly, it is argued that as there is variations in existence, there seems to be degrees of intensity or weakness for non-existence in Sadra’ s philosophy. In a sense, in one realization of existence, there is no non-existence while in a weak version of existence, non-existence is pervasive. Without an understanding of nonexistence, it is not possible to understand other philosophical concepts including paradox principle, authenticity of existence, variations in existence, substantive motion, and selfevolution.