It is well known that Husserl considered phenomenology to be First Philosophy—the ultimate science. For Husserl, this means that phenomenology must clarify the ultimate phenomenological-epistemological principle that leads to ultimate elucidation. But what is this ultimate principle and what does ultimate elucidation mean? It is the aim of this paper to answer these questions. In section 2, we shall discuss what role Husserl’s principle of all principles can play in the quest for ultimate elucidation and what it means for a principle to be ultimately elucidating (letztaufklärend) and ultimately elucidated (letztaufgeklärt). We will see that the Husserlian thesis that originary presentive intuitions are an immediate and the ultimate source of justification qualifies as the ultimate epistemological principle.