Being and Time (Heidegger)

"Being" cannot be derived from higher concepts by definition, nor can it be presented through lower ones. But does this imply that "Being" no longer offers a problem? Not at all. We can infer only that "Being" cannot have the character of an entity. Thus we cannot apply to "Being" the concept of definition as presented in traditional logic, which itself has its foundation in ancient ontology and which, within certain limits, provides a quite justifiable way of defining "entities." The indefinability of "Being" does not eliminate the question of its meaning; it demands that we look that question in the face.

-- Heidegger, from Being and Time