The Dasein is the only being who asks itself about its own being. That's why, as Heidegger says at the beginning of Being and Time, if we want to get a right knowledge about the Being, we should first analyze the being of the Dasein. But in order to do this properly, we must bear in mind that the Dasein is not a lonely being. If we wish to understand the ontological structures which make up human existence, we cannot limit ourselves to a solipsist analysis, but we must take into consideration the fundamentally mundane character of the Dasein. For Heidegger, the Dasein is an entity that is in the world. So the study of the structures of human life must involve the prior investigation of this "being-in-the-world". This need responds to one of the axioms which supports all the philosophical arguments of Being and Time: there is no essence of man that we have to discover, but it is the existence of the Dasein that is formed in a constant process of historical gestation. This is the way that Heidegger uses to introduce temporality in the constitution of the Dasein.