Martin Heidegger published *Sein und Zeit* in 1927, as the manuscript he submitted for his Habilitation at the University of Marburg. The work covers only the first half of a planned six-part book; the second half was never written. Even so, this 'half-book' is regarded as the most influential work of twentieth-century Continental philosophy.

Heidegger's question is the question of ancient philosophy: 'What does it mean to be?' Aristotle opened the question, but in Heidegger's view Western philosophy has always answered it through 'beings' (objects), forgetting the question of being itself. Heidegger reopens it, but from an unexpected starting point: the only being that can ask about being, the human, which he calls by a term of art, *Dasein* ('being-there').

The analysis of Dasein is the centre of the book. Dasein is not an object among objects; it is always 'being-in-the-world' (in-der-Welt-sein). We handle tools, communicate with others, plan for the future — these are modes of Dasein's being. But Dasein is also something else: it is always open to its 'possibilities', it constitutes its own being. At this point Heidegger separates 'authentic' (Eigentlichkeit) from 'inauthentic' (Uneigentlichkeit) existence. Inauthentic existence is lost in 'das Man' — doing what everyone does; authentic existence is taking up one's own possibilities, especially in the face of death.

Death is the key concept of the book: Dasein is 'being-toward-death' (Sein-zum-Tode). Death is not a moment but the horizon limiting my being at every moment. Authentic existence is the acceptance of this limit.

Heidegger briefly joined the Nazi Party in 1933 and gave speeches as rector of the University of Freiburg — for the rest of his life he was both pressed for an account of this and slow to give one. That political legacy has always shadowed readings of the book. Even so, Sartre, Levinas, Arendt, Derrida — and in Turkey, Nermi Uygur — did not build their own philosophies without reckoning with Heidegger.