In general, the Sartrean concept of the subject as "being-for-self" and "being-for-others" is read as if Sartre had sketched these structures as given "a priori" and therefore as unalterable. One of the consequences of this interpretation lies in calling Sartre's theory contradictory, especially with regard to his ethics, because of the assumption that, based on this concept, changing the inauthentic structures of the subject into authentic ones would be impossible. Contrary to this interpretation, I argue that Sartre's philosophical theory is by no means contradictory, neither in its relation to ethics nor as it relates to the complete edition of Sartre's philosophical writing, if one tries to understand what kind of theoretical requirements Sartre considered to be relevant and necessary. From this point of view, it is possible to work out an adequate and consistent interpretation. In order for me to argue for the immanent consistency of Sartre's theory and for the resulting possibility of an ethical theory based on it, I will discuss some aspects of the relation between epistemological, ontological and ethical elements within Sartre's philosophical system.