A human being is spirit. But what is spirit? Spirit is the self. But what is the self? The self is a relation that relates itself to itself or is the relation's relating itself to itself in the relation; the self is not the relation but is the relation's relating itself to itself. A human being is a synthesis of the infinite and the finite, of the temporal and the eternal, of freedom and necessity, in short, a synthesis. A synthesis is a relation between two. Considered in this way a human being is still not a self.... In the relation between two, the relation is the third as a negative unity, and the two relate to the relation and in the relation to the relation; thus under the qualification of the psychical the relation between the psychical and the physical is a relation. If, however, the relation relates itself to itself, this relation is the positive third, and this is the self.
Søren Kierkegaard, The Sickness Unto Death: A Christian Psychological Exposition for Upbuilding and Awakening
relation, self, spirit