Why do I have to be ethical? That is the essential question of a logical foundation of ethics in the phenomenology of Edmund Husserl. This article proposes to see the basic motivation of an ethical reason in the relationship between the two fundamental poles, that is the «Lifeworld» («Lebenswelt») and the «I-subject» («Ich-Subjekt»). This connection will be considered to constitute ethics in this article. This kind of ethics as a «condition of possibility» is then an a-priori ontological necessity. The article (...) will demonstrate how the composition of Husserl’s Prolegomena and his argumentation are an example of a foundation for phenomenological ethics: in this book Husserl derived logic as the first «condition of possibility». With logic’s three main characteristics — theory, normativity and praxis — it is the theoretical basis of a phenomenological ethics. (shrink)