This study attempts a reconstruction of the philosophy of the unfinished Opus tripartitum of Meister Eckhart. One new feature is the 'hermeneutic approach' to the question of Eckhart's philosophy, another is the recognition that the concept of Unity plays a decisive role in the organisation of his metaphysics, to the extent, indeed, that one can speak of a metaphysics of the One. Eckhart's metaphysics is determined to a contemplation of the divine, which in this thinking of Unity is understood as the principle and end of all things. Yet, at the same time Eckhart's thinking of Unity marks the boundaries of the ratio naturalis. In this way his thought combines a supreme trust in the effectivity of reason with a negative theology of the purest sort. By a systematic reading of linked texts of Meister Eckhart the trail of his philosophy of the One is followed down into the German sermons.