The article discusses rationalistic and existential approaches to the problem of existence. The comparison of Sartre’s pre-reflective cogito and Descartes’ reflective cogito makes it possible to define how Sartre’s thought moves from the thing to consciousness and from consciousness to the thing. At the same time, in Being and Nothingness Sartre does not only define the existence of the thing in its passivity—which in many respects corresponds to Descartes’ philosophy, but also as an open orientation towards consciousness, the latter concept not being fully developed by him. This statement may be regarded as a hidden component of Sartre’s key thesis about the role of the Other in the verification of our existence. The most important factor in understanding this is the concept of the look. Detailed analysis of Sartre’s theses in Being and Nothingness enables us to demonstrate that the concept of the look makes it possible to consider the identity of being-in-itself and being-for-itself (consciousness).