Existence is a matter of human and it exposes itself in different situations, preferences, and attitudes of human. In this regard, the emphasis that comes into prominence is that every human being has an existence that peculiar to his or her. Therefore, qualities of existence of every human are different from each other. This kind of approach faces us to some challenges while we are defining what we call "human". We can express these are; How can we understand each individual? What will be our foundation while we try to understand? Which one is more appropriate the realist and idealist systems? Or Should we understand individual in his/her existence? What is the meaning to oppose the general and abstract definitions such as "Human being is an animal who thinks", "Man is a social being"? Another context is the issue that whether there is a higher reality which become a source to the existence. Such a debate inevitably encounters us with the problem that distinction of the essentialist and non-essentialist existence. Is there a rational definition that everyone will understand for "Individual existence", "being for itself", "Dasein"? If there is no, shall we see them as irrational? We will try to deal with all the issues in this article.