Why did Martin Heidegger claim that existentialism was not a type of humanism?
In going back to Presocratic thought, Heidegger concluded that the original concern of man, or Dasein (in a cultural line that linked contemporary Germans to ancient Greeks), was Being. Heidegger believed that the Presocratics had only started to formulate the primary questions concerning Being, when the Socratics introduced a subject-object kind of metaphysics that already foreclosed one kind of answer to the original question of Being. Heidegger makes it clear to the reader that he does not know what this original question concerning Being was. Indeed, he devoted his philosophical work to trying to reconstruct the question, thereby inviting readers to ponder the same problem he did, with no conclusive answer. In this sense, Heidegger provides an exercise in meditation to those of his readers who take the time to understand him.
Heidegger wrote much about what that question might be, relying on a phenomenological intuition that “language is the house of Being.” He did not mean by this the language of “the they,” or even the discourse of French existentialists, such as Jean-Paul Sartre (1905–1980), with its insufficiently general concerns. Until the question of Being could be formulated, the kind of humanism that existentialism could be could not even be properly imagined, according to Heidegger.