Søren Kierkegaard (May 5, 1813–November 11, 1855)
The unhappy person is one who has his ideal, the content of his life, the fullness of his consciousness, the essence of his being, in some manner outside of himself. The unhappy man is always absent from himself, never present to himself. But one can be absent, obviously, either in the past or in the future. This adequately circumscribes the entire territory of the unhappy consciousness… The unhappy one is absent… It is only the person who is present to himself that is happy.