My main objective in the presented thesis is to explain two ways of defining what Heidegger calls the theoretical attitude in Being and Time. Furthermore, I try to establish a relation between these two definitions by distinguishing different ways of understanding Heidegger s position concerning Descartes. The thesis is divided in three parts. In the first part I present what I call the deficient definition of the theoretical attitude and how this definition is related with a demolition of Descarte s position. In the second part I show the problems of this definition and how this way of understanding Heidegger s position concerning Descartes is insufficient. In the third part I present what I called the positive definition of the theoretical attitude and how that this definition is related with a destruction of Cartesian thought. Destrucution primarily means to expose the fundaments.