A human being is "a self which relates itself to itself" and which has been "established by another." Two forms of despair are possible for such selves: despair not to will to be oneself, and despair to will to be oneself. The final paragraph of Part I.A.a. defines the condition of a self that is not in despair as a condition in which the self "in relating to itself and in willing to be itself" develops a "transparent" relationship with "the power that established it."
Part I.A.b. shows that despair is at once a distinction and a curse. Despair is a distinction because it is possible only for spiritual beings. It is not possible for animals (which do not have free spirits), nor for the non- Christians of the distant past (who were not aware of themselves as free spirits that could attain eternal life). Nevertheless, despair is a condition of awful unhappiness and frustration.
It is immensely difficult to overcome despair. Whereas physical sicknesses are caught at a discrete time and then endured, despair is a spiritual condition that one is continually catching unless one is continually rooting it out.
Part I.A.c. elaborates on the torments and complexities of despair. For Christian people, who are aware of eternal life, physical illness is not the "sickness unto death." The sickness unto death for them is worse. If Christian people do not attain eternal life, the alternative is a condition of eternal death--a condition in which one continues to exist even though one is dying or wants to die.
Part I.A.c. also offers two down-to-earth examples of despair. The first example is a person who wants to be Caesar but fails to accomplish this goal. This person appears to be despairing over something (over not being Caesar). In fact, however, he is despairing over himself: he wishes that he were something that he is not (Caesar), and he wishes that he were not himself (since he is not Caesar). The second example makes the same point. A girl whose lover has died or has betrayed her may appear to be despairing over the lover, but in fact she despairs over herself; she wishes that she were still her lover's beloved.
The final three paragraphs return to the point that despair is an eternal condition. Whereas physical illness ends in physical death, the spiritual sickness of despair torments the spirit without killing it.