In the philosophy of existence, even the definition of the word "existence" constitutes a pretty difficult problem and advocates of such philosophy themselves have not succeeded in finding a proper definition which would be satisfactory to all of them. However, in short, I believe I can define existence as "total-subjective being of man." Here "total being" must bot be construed as an absolute, perfect and eternal entity. On the contrary, "total being" indicates existence as a finite being which is fully aware of its finiteness. As Blaise Pascal pointed out in his "Pensees sur la Religion," man is hung halfway between being and nothingness. The position halfway between being and nothingness is, in German philosophical terminology, called "Grenzsituation." I mean by "total being" man's existence seen in its Grenzsituation. Now "subjective being" indicates existence as ceaseless projection of itself toward the future. Human existence is an effort to pull itself toward the future. Existence is always thrust toward the cliff of nothingness and, if it should cease projecting itself toward the future, it would soon be engulfed into nothingness. Man must be constantly vigilant in his effort of projecting himself toward the future. Like Jesus who kept His last vigil praying at Gethsemane while His disciples had fallen asleep, man must be on the watch for the future (Leo Shestov, La Nuit de Gethsemani). Thus I mean by "subjective being" man's existence in its ceaseless effort of projecting itself toward the future. Man as a finite being cannot escape death, illness, suffering, conflict, sin and so forth (Grenzsituation). Existence to be analyzed by the philosophy of existence is human being in this Grenzsituation. But we must note that human being finds itself also in history. In this sense, human being is a historical existence. Any existence that finds itself in history is subject to change in accordance with the principle of sufficient reason. Even the Grenzsituation changes its meaning in history, in as much as the limitation of man is in a sense historical. Grenzsituation, expressed in another way, is the forfeiture of man's self. Man's forfeiture of self occurs in history and its meaning changes with the history. When we understand existence as historical existence which is subject to change, we can override our limitations and elevate ourselves above the pit of nothingness. Philosophy of existence in the form thus far conceived does not seem to have grasped the meaning of Grenzsituation from the viewpoint of historical existence. My monograph is an attempt to clarify the meaning of "existence" from various angles by casting light on it from the viewpoint of historical existence.