Full Idea
Dasein's Being-what-it-is (essentia) must….be conceived in terms of its Being (existentia).
Gist of Idea
Being what it is (essentia) must be conceived in terms of Being (existence)
Source
Martin Heidegger (Being and Time [1927], 67/42), quoted by Richard Polt - Heidegger: an introduction 3.§2
Book Reference
Polt,Richard: 'Heidegger: an introduction' [Routledge 2003], p.30
A Reaction
This seems to be the origin of Sartre's famous slogan 'existence before essence'. It seems to be a rebellion against Husserl's quest for essences.
Related Idea
Idea 15570 Phenomenology is the science of essences - necessary universal structures for art, representation etc. [Husserl, by Polt]