The human being is a synthesis: finite and infinite, temporal and eternal, freedom and necessity are its constitutive poles. But the synthesis is not given; it must be held together, and the holding-together is itself a third thing — the self. This layered structure (the synthesis, the relation that holds the synthesis together, the self as this relation) means that the self is always an achievement, never a possession. A human being does not have a self the way they have a body; they either are a self or they are failing to be one, and this condition is dynamic, unstable, and requiring constant renewal.
Anti-Climacus insists that the self has a measure — a standard against which it is assessed — and that this measure increases as one's awareness of it increases. The self before society has one measure; the self before God has an infinite measure. This means that a person who lives entirely within social categories — measuring themselves by social success, by the opinions of others, by the conventions of their time — has a lower measure of selfhood than one who stands before God and is assessed by the infinite. The higher the measure, the more clearly despair can be seen; the more clearly despair can be seen, the more acute the possibility of genuine faith.
The formula of faith that Anti-Climacus offers is exact: "In relating itself to itself and in willing to be itself, the self rests transparently in the power that established it." Faith is not primarily an intellectual assent to doctrinal claims; it is a mode of existing — the mode in which the self, having passed through the school of despair and having recognised its own groundlessness, allows itself to be grounded in God rather than in its own projects or the social world. The "transparent" in the formula is important: faith is not blind surrender but clear-eyed acknowledgement of both the self and the power in which it rests.
The analysis of the self appears in Part One of The Sickness Unto Death. Anti-Climacus's definition — "The self is a relation that relates itself to itself and in relating itself to itself relates itself to another" — is one of the most commented-upon sentences in nineteenth-century philosophy, and its influence on twentieth-century existential and phenomenological thought was profound.
