What's the difference between ontology and existentialism?

Question: What's the difference between ontology and existentialism!?
Best Answer - Chosen by Asker:

Ontology is the metaphysical study of what exists and why!. And we're talking at a very nuts and bolts level!.

Existentialism is a philosophical system!. As such, it includes a number of metaphysical claims, some of which are ontological in nature and some of which are not!.

One of existentialism's classic ontological claims is that 'existance precedes essence' - things exist before they have a nature and a purpose!.Www@QuestionHome@Com

They share the key word "existence" but they are very different!.

Ontology is study of being or existence, basic forms of existence, relationships between them, etc!.

Existentialism is a philosophical movement that centers around the idea that individuals create their own meanings of life!.

Ontology is a science!. Existentialism is a school of philosophy!.Www@QuestionHome@Com

ontology is a word that refers to stratum or levels of existence, and existentialism is the study of the nature of existence and its purpose or meaning!.Www@QuestionHome@Com

They are both quite similar my friend!.!.!.!.
The subject of ontology is the study of the categories of things that exist or may exist in some domain!. The product of such a study, called an ontology, is a catalogue of the types of things that are assumed to exist in a domain of interest D from the perspective of a person who uses a language L for the purpose of talking about D!. The types in the ontology represent the predicates, word senses, or concept and relation types of the language L when used to discuss topics in the domain D!. An uninterpreted logic, such as predicate calculus, conceptual graphs, or KIF, is ontologically neutral!. It imposes no constraints on the subject matter or the way the subject may be characterised!. By itself, logic says nothing about anything, but the combination of logic with an ontology provides a language that can express relationships about the entities in the domain of interest!.

An informal ontology may be specified by a catalogue of types that are either undefined or defined only by statements in a natural language!. A formal ontology is specified by a collection of names for concept and relation types organised in a partial ordering by the type-subtype relation!. Formal ontologies are further distinguished by the way the subtypes are distinguished from their super-types: an axiomatized ontology distinguishes subtypes by axioms and definitions stated in a formal language, such as logic or some computer-oriented notation that can be translated to logic; a prototype-based ontology distinguishes subtypes by a comparison with a typical member or prototype for each subtype!. Large ontologies often use a mixture of definitional methods: formal axioms and definitions are used for the terms in mathematics, physics, and engineering; and prototypes are used for plants, animals, and common household items!.
Existentialism is a philosophical movement which emphasises on individual existence, freedom, and choice!.
There are several philosophical positions all related to existential philosophy but the main identifiable common proposition, is that existence precedes essence!. By this, existentialism states that man exists and in that existence man defines himself and the world in his own subjectivity, and wanders between choice, freedom, and existential angst!.

The first philosopher to use the term was Soren Kierkegaard (1813 - 1855), who reacted against systematic rational philosophy, specially Hegel, and grasped the notion of a truth inside of the evolving self!.

Deriving from this stress on existence there are other main subjects and images that have been developed by the existentialists!.

Becoming a Being (existential ontology):
We are what we can become!. Ours is a process, and our becoming is our ontic possibility of becoming!. Human existence is a project, in which past and present are subordinate to future, is the main residence of our existence, because it is the north of our projection of ourselves!. “Human existence cannot have a relationship with being unless it remains in the midst of nothingness!.” (M!. Heidegger - was ist metaphysik!?)!.
Nothingness
Nothingness appears in existentialism, as the placeholder of the possibility!. The awareness of anything in the world that is not my own existence (which by the way,cannot be held in consciousness without being nihilized) is an awareness of nothingness, that is, what I, this existence am not and in some cases I could become!.
Absurd
We arrive from nothingness to absurd at the moment that we ask for a meaning after we have become aware of the other (trough the prior explained negation)!. Absurd is a leit motiv in existentialism, specially in Sartre and Camus!. It is sometimes possible to overcome absurd, with absurd itself, as Camus says in The Myth of Sisyphus: "The struggle itself toward the heights is enough to fill a man's heart!. One must imagine Sisyphus happy!."
Ethics / Subjectivity / Good faith
Our view of the world is enough to become Truth, because it is based on our facts!. What we do with this truth, depends on our good or bad faith, that is, the ability to act as if in our act the entire mankind would be represented!. That is enough to prove an act as an ethical one!.
Choice
We always have a choice!. Existentialism does not stand for any kind of determinism except the one that determines our individual facts (existence)
We choose, and in choosing (in good or bad faith) we define ourselves!. Choice is a definition of an existence in the world, towards an object outside of itself!.
Choice is all that we have, without confirmation of our act; we never know what was right to choose!. The doubt of our acts, together with the contingence of existence, leads to
Angst
The main characteristic of existence itself, when we face our contingence, and the absurdity of our acts and choices:
For Heidegger, it is that trough which fear becomes possible!. For Kierkegaard is a desire for what one fears!. For Sartre, it is the immediate consequence of facing the possibility of nothingness!.Www@QuestionHome@Com