The Sickness Unto Death (Danish: Sygdommen til Døden) is a book written by Danish philosopher Søren Kierkegaard in 1849 under the pseudonym Anti-Climacus. A work of Christian existentialism, the book is about Kierkegaard's concept of despair, which he equates with the Christian concept of sin, which he terms, "the sin of despair."
Anti-Climacus introduces the book with a reference to John 11:4: "This sickness is not unto death." This quotation comes from the story of Lazarus, in which Jesus raises a man from the dead. However, Anti-Climacus raises the question: would not this statement still be true even if Jesus had not raised Lazarus from the dead? While the human conception of death is the end, the Christian conception of death is merely another stop along the way of the eternal life. In this way, for the Christian, death is nothing to fear. Instead, the inability to die is what is to be feared. The true "Sickness unto Death," which does not describe physical but spiritual death, which stems from not embracing one's self, is something to fear according to Anti-Climacus.
This sickness unto death is what Kierkegaard calls despair. According to Kierkegaard, an individual is "in despair" if he does not align himself with God or God's plan for the self. In this way, he loses his self, which Kierkegaard defines as the "relation's relating itself to itself in the relation." Kierkegaard defines humanity as the tension between the "finite and infinite", and the "possible and the necessary", and is identifiable with the dialectical balancing act between these opposing features, the relation. While humans are inherently reflective and self-conscious beings, to become a true self one must not only be conscious of the self but also be conscious of being grounded in love, viz the source of the self in "the power that created it." When one either denies this self or the power that creates and sustains this self, one is in despair.
There are three kinds of despair presented in the book: being unconscious in despair of having a self, not wanting in despair to be oneself, and wanting in despair to be oneself. The first of these is described as "inauthentic despair," because this despair is born out of ignorance. In this state, one is unaware that one has a self separate from its finite reality. One does not realize that there is a power that created and continues to create one, and accepts finitude because one is unaware of the possibility of being more inherent in selfhood. The second type of despair is refusing to accept the self outside of immediacy; only defining the self by immediate, finite terms. This is the state in which one realizes that one has a self, but wishes to lose this painful awareness by arranging one's finite life so as to make the realization unnecessary. This stage is loosely comparable to Sartre's bad faith. The third type is awareness of the self but refusal to acknowledge one's dependence on love, i.e., the power that created one. In this stage, one accepts the eternal and may or may not acknowledge love, but refuses to accept an aspect of the Self that one in reality is, that is to say, the self that one is in love. Kierkegaard identifies this type of demonic despair as the most heightened form of despair.
To not be in despair is to have reconciled the finite with the infinite, to exist in awareness of one's own self and of love's power. Specifically, Kierkegaard defines the opposite of despair as faith, which he describes by the following: "In relating itself to itself, and in willing to be itself, the self rests transparently in the power that established it." People commonly ascribe the name "God" to the "power that created" the self, but Anti-Climacus's text is more subtle than this orthodox viewpoint. Kierkegaard certainly was thinking of God, but what it means to have a personal relation with God, and how God is love are the real subjects of this book. While the book is, in many ways, a phenomenology of prayer; it is just as much a phenomenology of what a Romantic-despite-himself could offer to the future of human maturity by way of a relational view of the self as grounded in creative love.
The Sickness Unto Death has strong existentialist themes. For example, the concept of the finite and infinite parts of the human self translate to Heidegger's concept of 'facticity' and Sartre's concept of 'transcendence' in Being and Nothingness. Kierkegaard's thesis is, of course, in other ways profoundly different from Sartre, most obviously because of Kierkegaard's belief that only religious faith can save the soul from despair. This particular brand of existentialism is often called Christian existentialism.
Some have suggested that the opening of the book is an elaborate parody of the often bafflingly cryptic philosophy of Georg Wilhelm Hegel; however, some scholars, such as Gregor Malantschuk, have suggested otherwise (Armed Neutrality and An Open Letter, Simon and Schuster, 1969, pp. 65–6 and n. 7 on pp. 165–6).