Written by one of the pre-eminent interpreters of Heidegger, this book is an important statement about the basis of human sociability that is a major contribution to the continuing debates about Heidegger in particular, and ethics in general. Existential philosophy is often thought to promote moral nihilism in which everything is permitted. This book demonstrates that, in the case of Martin Heidegger, any such accusation is unjust. On the contrary, Heidegger thought seriously about the implications of human co-existence, and this book shows that conceptions of trust and responsibility that lie at the very heart of morality are to be found in the sketch of Mitsein - our being together with one another in the world - offered in Being and Time. That Heidegger never developed these conceptions may explain why they have been overlooked, but renders them no less important for that.