A philosophical system popular in the 18th and 19th Century represented by a profound reaction against the traditional Western ontological position that humanity is defined by an essence, form, or substance that exists distinctly from a particular human being.
The phrase that most clearly encapsulates existentialist ontology is: “existence precedes essence” (Jean Paul Sartre, Being and Nothingness,1943). This assertion is designed to challenge the notion that we are born with a pre-established human nature, particularly in regard to our consciousness. We become conscious, at some point, and find ourselves in the world. As we make choices, we define our nature, or character as opposed to discovering an innate character (or soul) pre-existing in us. If this awareness of being “thrown” into the world is an accurate reflection of reality, it follows that each individual is radically free to make choices which in turn define who they are.
But the idea that being precedes essence has other metaphysical implications. Existentialists (with the notable exception of Soren Kierkegaard) take the absence of a human essence to reflect a universe absent of any significant essence, e.g., God. Thus, our existence is not only free of a pre-existing human essence, it is also free of any divine essence that would create or impose necessity on our existence. Therefore, since we, as finite beings, exist in the absence of both human and divine essence, there is no necessity to our existence at all. That is, there is no more reason for us to exist than for us not to exist; our existence is absurd (lacking transcendent value).
The normative moral view resting on the metaphysical assertion that "being precedes essence" maintains that moral obligation is created or arises out of the choices we make as absolutely free agents. Actions have no intrinsic moral value, nor do we as individuals have intrinsic moral value, hence the only true virtue for the existentialist is authenticity. Authenticity is the disposition to acknowledge and embrace the choices we have freely made and to accept any obligation that might follow there from.