Martin Heidegger (1889-1976) was a German philosopher whose writings on phenomenological ontology had an important influence on the development of existentialism, philosophical hermeneutics, and twentieth century Protestant theology. His major works included Sein und Zeit (Being and Time, 1927), Was ist Metaphysik? (What is Metaphysics?, 1929), Identität und Differenz (Identity and Difference, 1957), and Was heisst Denken? (What is called Thinking?, 1968).

Being and Time (1927) investigates the meaning of being as defined by temporality, and it analyzes time as a horizon for the understanding of being. Heidegger develops a phenomenological ontology that is based on the hermeneutics of Da-sein (being-there). He uses the term "Da-sein" to describe a mode of being that understands its own being. Da-sein is conscious being, and it is the kind of being that belongs to human beings.

According to Heidegger, Da-sein has both an ontic (existential) and ontological priority over other kinds of being. It is a kind of being that may understand not only its own being, but also other kinds of being. Thus, the ontic and ontological structure of Da-sein may be the foundation for every other kind of being.

The being of Da-sein is different from the being of objective presence, because Da-sein may project its own possibility. The factuality of Da-sein includes projected possibility, and thus it is different from the factuality of whatever is objectively present.

Da-sein may be authentic or inauthentic, depending on whether or not its projected possibility belongs to itself. Authenticity (Eigentlichkeit) and inauthenticity (Uneigentlichkeit) are modes of possibility for Da-sein. Authenticity is the mode of being by which Da-sein reveals itself, but inauthenticity is the mode of being by which Da-sein conceals itself. Thus, authenticity and inauthentity are fundamental existential determinations of Da-sein.

The possible determinations of Da-sein and of other modes of being may vary in their ontological character. Heidegger refers to determinations of Da-sein as "existentials," while he refers to determinations of being unlike Da-sein as "categories." Existential determinations do not belong to "things at hand" or "things objectively present," and they are instead constituents of the being of Da-sein. Categorial determinations, on the other hand, belong to "things at hand" and "things objectively present."

Heidegger distinguishes between "being at hand" (Zuhanden-sein) and "being objectively present" (Vorhanden-sein) as different modes of categorial determination. "Things at hand" (Zuhandenen) are encountered by Da-sein in its concern with, or in its taking care of, the world. Zuhandenen may be useful or "handy." "Things objectively present" (Vorhandenen) may also become objects of concern for Da-sein. Vorhandenen are important constituents of the actuality and reality of the world. While "handiness" (Zuhandenheit) is the being of beings that may be initially encountered by Da-sein, "objective presence" (Vorhandenheit) is the being of beings that are found to be of concern after Da-sein has encountered whatever is at hand.

Categorial determinations include handiness, objective presence, conspicuousness, obtrusiveness, obstinacy, thingliness, and remoteness. Existential determinations include temporality, spatiality, being-in-the-world, worldliness, nearness, disclosedness, thrownness, attunement, understanding, interpretation, significance, discourse, language, idle talk, curiosity, ambiguity, and falling prey.

Da-sein, according to Heidegger, is always being-in-the-world (In-der-Welt-sein). The world includes the totality of "things at hand" and "things objectively present," but it is also the realm in which Da-sein has its being. The world is a constitutive factor of Da-sein. The world is a structural component of being-in-the-world.

Da-sein as being-in-the-world has both temporality and spatiality. The spatiality of Da-sein includes de-distancing and directionality. De-distancing is a mode of bringing beings nearer to each other. Da-sein is a mode of being that de-distances being in space. It is also a nearness (Nähe), rather than a remoteness (Entferntheit). The nearness that is determined by Da-sein has a character of directionality.

How can the "there" of being-there be defined or analyzed? Da-sein is disclosed to itself by attunement (Befindlichkeit), understanding (Verstehen), and discourse (Rede). Attunement is a mode by which the mood and thrown possibility of Da-sein are disclosed to itself. Understanding is a mode by which the meaning and significance of being-in-the-world are disclosed to Da-sein. Discourse is the mode by which the intelligibility of Da-sein may be communicated.

The "there" of being-there may be disclosed by attunement and understanding. It is also the thrownness of its being, because Da-sein always discovers that it is already in-the-world. Da-sein is attuned to itself when it discovers its own mood, and when it discovers the thrownness of its own being.

Da-sein is factical, says Heidegger, because it is thrown being-in-the-world. It understands its own thrownness by projecting itself as factical potentiality. Facticity or thrownness is the "there" of Da-sein. Disclosedness may be authentic or inauthentic. Da-sein may be disclosed to itself authentically or inauthentically in relation to its own existence and thrown potentiality.

Truth is the disclosedness of Da-sein. There can be no truth without Da-sein. The full disclosedness of Da-sein is based on its concern with being-in-the-world.

Heidegger describes fear as a mode of attunement, and he says that it may be caused by something that is at hand or by something that is objectively present. Fear may become alarm, horror, or terror. Fear concerning the well-being of others may be a mode of attunement to others.

Heidegger also makes a distinction between fear and Angst. Fear (Furcht) may be caused by something definite, by something that is at hand, or by something that is objectively present. Angst may be caused by something indefinite, by something that is not at hand, or by something that is not objectively present. Fear may be an apprehension of an innerworldly being, but Angst is not an apprehension of an innerworldly being. Fear may cause flight from something that is at hand or from something that is objectively present, but Angst may cause flight from Da-sein itself.

Da-sein may be disclosed to itself as a projection of itself toward its own potentiality. The freedom of Da-sein to understand its own potentiality may also be the freedom to project itself authentically or inauthentically. Da-sein is free to project or not project itself. It is free to reveal or conceal itself.

Da-sein reveals itself as care, not only in its being-with "things at hand" and in its being with "things objectively present," but also in its being-in-the-world. Thus, being with "things at hand" or with "things objectively present" means being concerned with, or taking care of, them. Care includes taking care of things at hand, taking care of things objectively present, and taking care of Da-sein itself. It brings things nearer to Da-sein. It is circumspect whenever it discovers things at hand, and it is heedful whenever it discovers not only things at hand, but also things not at hand.

Thus, Da-sein takes care of things, and it takes care of other beings. It takes care of being and time. It is fundamentally concerned about its mode of being, and it thus becomes attuned to projecting its own potentiality. Being-with-others, having concern for others, and taking care of the world are modes by which Da-sein becomes attuned to being-in-the-world. Thus, its being reveals a care (Sorge) and concern (Fürsorge) by which it understands and transcends itself.

Heidegger describes conscience (Gewissen) as a call of care (Ruf der Sorge) that summons Da-sein to return from falling prey to the world. Falling prey to the world is a form of inauthenticity, because Da-sein becomes absorbed by being with others, by being with "things at hand," and by being with "things objectively present," to the extent that it no longer reveals itself. Conscience enables it to recognize whatever is lacking in its own being and to redirect itself toward its full potentiality. The call of conscience is to take care of other beings and of being-in-the-world.

Resoluteness (Entschlossenheit) is the mode by which Da-sein is disclosed to itself as wanting to act according to conscience. Resoluteness is a willingness of Da-sein to project itself into situations in which it may experience Angst. Resoluteness is a freedom from fear, and it is an acceptance of Angst as an existential possibility. It is authentic being-in-the-world, because Da-sein takes care of things, of other beings, and of its own mode of being.

Heidegger describes anticipatory resoluteness as being-ahead-of-itself (Sein sich vor Weg). Anticipation refers to the "not yet," but thrownness refers to the "already projected." Authentic existence is resolute, but inauthentic existence is irresolute.

Being-toward-death (Sein zum Tode) is attunement to no longer being-in-the-world. Authentic being-toward-death is attunement to death as an existential possibility. Inauthentic being-toward-death is a lack of attunement to death as an existential possibility. Being-toward-death is Angst insofar as it is an attunement to death as a negation of the individualized being of Da-sein. Angst may arise when Da-sein is faced with the possible annihilation of its own existence.

Da-sein is a temporal mode of being, says Heidegger. Authentic temporality is the being of Da-sein, but inauthentic temporality is the being of innerworldy things and of beings unlike Da-sein. Time is a structural factor for Da-sein. Temporality makes possible the historicity (Geschichtlichkeit) of Da-sein, which may remain undisclosed or be discovered by historical inquiry. The historicity of Da-sein also implies that ontological inquiry has its own historicity.

Da-sein understands itself by projecting itself as thrown possibility. The thrownness of Da-sein is its "having been," but the projected possibility of Da-sein is its "already being" and its "not yet." Thus, Da-sein unifies the past, present, and future. The past, present, and future are referred to by Heidegger as the "ecstacies" of temporality. Temporality is "ecstatic," and it is the meaning of being-there. Da-sein temporalizes itself in its being-in-the-world, and it reveals the "ecstatic" unity of temporality.

Heidegger concludes that the distinction between Da-sein and other modes of being is only the beginning of ontology, and that temporality as the meaning of being still needs to be further explained. If temporality is the meaning of being, then what is the meaning of temporality? Heidegger does not fully answer this question, but leaves it open for further investigation.

Martin Heidegger. Being and Time. Translated by Joan Stambaugh. Albany: State University of New York Press, 1953.