Existentialism is a chiefly 20th century philosophy that emphasizes existence for its own sake alone, disregarding God and morality and almost anything else. Rene Descartes famously declared, "I think therefore I am"; but an existentialist might exclaim, "I am therefore I think." Arbitrary actions are a favorite of existentialists, as they resist moral codes of behavior.
Existentialism denies the dependency of human existence on God and views human existence as an independent and meaningless form. It asserts that we do not have fundamental attributes, such as morality or a soul, and that humans develop these attributes in a meaningless manner as their lives progress. Beyond that, existentialism means different things to different people and thus can often be the subject of debate by students of atheistic philosophy.
The name derives form one of its central concepts, the notion that a human being's "existence precedes essence."
The earliest existentialist was Søren Kierkegaard. Other writers were Karl Jaspers and Martin Heidegger. The movement did not, however, develop its name or popularity until the 20th century when it was popularized by Jean Paul Sartre and Simone de Beauvoir. 
Especially Sartre says, "Existence precedes essence." He asserts that man appeared not from essence (God or an idea) but rather from nothing and then defined himself and gave essence to himself.
The writings of existentialists such as Kierkegaard, Sartre and Camus reflect an epistemology that is not only based upon the criteria of rationalism or empiricism. Theirs is an appeal to the intuitive, as reflected by Sartre’s references to the “nausea” stirred in him by the contradictions and hypocrisy in human behavior. The same appeal to the intuitive can be found in Soren Kierkegaard’s notion of dread and in Albert Camus’ reflections on universal guilt.