“Understanding the other” is the paradoxical point of ethics: without it no ethics is possible, and with it I would take away the other it’s alterity. Modern philosophy involves us in such a paradoxical situation, especially in the opposition of Husserl and Levinas. Following their thoughts, we would come near its deep roots. We will show post-structuralism, by decentering of subject, would help us to get rid of this paradox. In Kant’s words, we want to find transcendental condition of ethics. At the end, we will reveal a new aporia concealed in every relation with the other, an Aporia, which involves post-structuralism too.