The political actions of Martin Heidegger raise a compelling question to those concerned with philosophy: How was one of the most important philosophers of the 20th century willing to ally himself with Nazism and what does this mean for philosophy? This question has been raised and brushed aside from the end of the Second World War, when Heidegger was formally accused for his involvement with Hitler's regime and forbidden to attain any official teaching position henceforth. Important thinkers, like his colleague and friend in the pre-war years, Karl Jaspers. testified then against him. But Heidegger's fame rose again above his indignity in the 1950s, when Hannah Arendt in the USA and Jean-Paul Sartre in France, among others, began once again to spread the word of his philosophy and translate his works. These problems have recently intensified after the posthumous publication of Heidegger's Black Notebooks (Schwarze Hefte) in 2014, containing his notes from 1931-1941. Their publication caused seismic tremors in the field of philosophy, with Heidegger's opponents considering themselves finally vindicated by his own pen, and his supporters trying to separate thinking from actual life. However, the influence of Heidegger's thought - as well as that of other representatives of German anti-humanist nationalistic conservatism, such as Oswald Spengler - continues to spread, not only in academic but also in broader social and political circles. In the face of this spreading influence, which is combined with the influence of some disciples such as Nick Land or Giorgio Agamben, a biographical-type confrontation is not enough. We should confront Heidegger directly at the core of his philosophy. We cannot afford to ignore him or bury him under superficial gossip. Instead, we should confront him philosophically. We might be able to clear the ground for such a confrontation, if we resort to the philosophical concepts of autonomy, temporality, and human freedom. In my opinion, the most important philosophical challenge to Heideggerian thought, in this context, has been expressed by Cornelius Castoriadis. In the following chapters, I will argue in favor of this position, outlining the Castoriadian philosophy of time in contrast to the Heideggerian and highlighting the main points of their opposition. The book is divided into two parts. Part one, "Castoriadis against Heidegger" deals with the basic concepts and arguments of both philosophers around human existence, the individual, society, and temporality. We start by presenting Heidegger's scandal in the social-historical context but also with the help of Hannah Arendt's last assessment of the thinker. We move on to outline the basic concepts and structures of Heidegger's philosophical thinking in parallel with Castoriadis' criticism as regards politics, ontology and technoscience. Part two, "Time and existence" first examines the relevant approaches of Lukacs and Ricoeur in comparison to Heidegger and Castoriadis, to elucidate the basic concept of temporality and then presents an outline of Castoriadis' insights on time and existence, before finally applying these concepts to contemporary social-historical phenomena that redefine and transform human existence and social temporalities, like space exploration, digital cyberspaces and online communications. "This book is a tour de force that excites those who agree or disagree with the philosophy of Heidegger. With clarity and sophistication, Schismenos revives the public debate to reveal the rich possibilities for another world. A must-read for everyone interested in a philosophy that matters." - VASILIS KOSTAKIS - Professor of P2P Governance at Tallinn University of Technology and Faculty Associate at Harvard University Alexandros Schismenos holds a Ph.D. in Philosophy from the University of Ioannina. He is the co-author of Castoriadis and Autonomy in the Twenty-First Century (Bloomsbu