This article elaborates Heidegger's statement that "freedom is the ground of ground". The notion of freedom is a key concept in a period of Heidegger's thought that he himself has called it "metaphysics of Dasein". Freedom for him is not primarily a political discussion, nor a question of causality, but an ontological principle, an answer to the question of how the world can be opened to Dasein, and why things become questionable for man and on which ground is based his search for finding reasons, proofs and demonstrations. The freedom is "the ground" of our thinking by means of reasons. Heidegger's way of description how to achieve this foundation begins with Dasein's prior understanding of Being (Sein), which is the condition of possibility to be related to other beings. But the possibility of understanding of Being (Sein) itself depends on the transcendence of all beings. In transcending, Dasein gets beyond beings and moves towards their totality, which is the world. The fundamental characteristic of the world is "for the sake of" (Umwillen). There is an intrinsic interrelationship between "for the sake of", which is the primary attribute of the world and being-in-the-world and the freedom of Dasein.