As a Christian, Kierkegaard belongs to the tradition of religious thinking—theology is too systematic a term—represented by Augustine, Pascal, Newman and Karl Barth, as distinct both from the Thomist tradition of official Catholicism and from the liberal Protestantism of men like Schleiermacher. As a secular dialectician, he is one of the great exponents of an approach, equally hostile to Cartesian mechanism and Hegelian idealism, to which the Germans have given the name Existential—though it is confined neither to Germans like Nietzsche, Jaspers, Scheler, Heidegger, but may be found, for instance, in Bergson and William James, nor to professional philosophers, for the same approach is typical of what is most valuable in Marx and Freud.
In contrast to those philosophers who begin by considering the objects of human knowledge, essences and relations, the existential philosopher begins with man's immediate experience as a subject, i.e., as a being in need, an interested being whose existence is at stake. He does not assert, as he is usually accused of asserting, the primacy of Will over Reason, but their inseparability. As Augustine says: "I am and know and will; I am knowing and willing; I know myself to be and to will; I will to be and to know." There is, therefore, no timeless, disinterested I who stands outside my finite temporal self and serenely knows whatever there is to know; cognition is always a specific historic act accompanied by hope and fear. To realize this is not, again, to abandon as hopeless the search for common sharable truth and surrender to a subjective relativism: on the contrary, it is precisely in the interest of such a common truth, that it is necessary for the individual to begin by learning to be objective about his subjectivity, "to know his station," to become conscious every time he asks calmly of an object or an event, "What are you?" of his urgent, simultaneous aside, "Be this. Don't be that."
From this viewpoint, the basic human problem is man's anxiety in time;e, g., his present anxiety over himself in relation to his past and his parents (Freud), his present anxiety over himself in relation to his future and his neighbors (Marx), his present anxiety over himself in relation to eternity and God (Kierkegaard).
Beautiful preface by W.H. Auden.