There are essentially two forms of illusion: that of hope and that of recollection. The adolescent’s illusion is that of hope, that of the adult recollection. But precisely because the adult suffers from this illusion, his conception of illusion itself is also quite the one-sided one that the only illusion is the illusion of hope. And that is understandable. What afflicts the adult is not so much the illusion of hope as, no doubt among other things, the grotesque illusion of looking down from some supposedly higher vantage-point, free from illusion, upon the illusions of the young.
The young person is illuded: he hopes for the extraordinary both from life and from himself. While as far as adult illusion is concerned, on the other hand, this is often found in the adult’s recollection of youth.
An older woman who has supposedly left all illusion behind is often found to be fantastically illuded, as much as any young girl, in her own recollections of herself as a young girl, of how happy she was then, how beautiful, etc. This fuimus [we have been] we so often hear from older people, is just as great an illusion as the young person’s illusions of the future; they lie or invent, both of them
Kierkegaard, The Sickness Unto Death
For Kierkegaard, this habit of self-delusion is a way of escaping the reality of our selves. Selfhood, or personhood, or existence are important questions for him and he challenges his reader to quit escaping from the self into the realm of fantasy, which is hope or recollection, and begin to explore existence as it really is.