  Heidegger's philosophical development began when he read Brentano and Aristotle, plus the latter's medieval scholastic interpreters. Indeed, Aristotle's demand in the Metaphysics to know what it is that unites all possible modes of Being (or ‘is-ness’) is, in many ways, the question that ignites and drives Heidegger's philosophy. From this platform he proceeded to engage deeply with Kant, Kierkegaard, Nietzsche, and, perhaps most importantly of all for his subsequent thinking in the 1920s, two further figures: Dilthey (whose stress on the role of interpretation and history in the study of human activity profoundly influenced Heidegger) and Husserl (whose understanding of phenomenology as a science of essences he was destined to reject). In 1915 Husserl took up a post at Freiburg and in 1919 Heidegger became his assistant. Heidegger spent a period (of reputedly brilliant) teaching at the University of Marburg (1923–1928), but then returned to Freiburg to take up the chair vacated by Husserl on his retirement. Out of such influences, explorations, and critical engagements, Heidegger's magnum opus, Being and Time (Sein und Zeit) was born. Although Heidegger's academic and intellectual relationship with his Freiburg predecessor was complicated and occasionally strained (see Crowell 2005), Being and Time was dedicated to Husserl, “in friendship and admiration”.