In Concluding Unscientific Postscript, the pseudonym Johannes Climacus explains, "[I]n asking ethically with regard to my own actuality, I am asking about its possibility, except that this possibility is not esthetically and intellectually disinterested, but is a thought-actuality that is related to my own personal actuality--namely, that I am able to carry it out."
In other words, the point of ethics is to act, rather than endlessly assess our options. At some point, I have to DO something. If we attempt "within possibility to distinguish between possibility and actuality... actuality and deception are equally possible... Only the individual himself can know which is which." That is, when considering possibilities, we cannot ignore those which are distasteful to us. This includes the possibility that we are entirely mistaken. This is why action becomes vital; only by lived decision can we truly distinguish our possibilities from our actualities.
Of course, the question of whether even the individual can really know the difference between possibility and actuality remains open, since if "deception can reach just as far as actuality" it is difficult to see how even the individual him/herself can be certain of their own actuality, as long as they are conflating actuality and possibility. Indeed, this dilemma will eventually be recognized by another Kierkegaardian pseudonym, Anti-Climacus, as constituent of two prevalent forms of despair--the despair of false possibility, and the despair of false actuality.
In light of this situation, Johannes reminds us that we must be vigilant: "When the esthetic and the intellectual inspect, they protest every esse that is not a posse; when the ethical inspects, it condemns every posse that is not an esse, a posse, namely, in the individual himself, since the ethical does not deal with other individuals.—In our day everything is mixed together." This is always the case in thought. There is always a mixing of categories, precisely because we will never, as mere humans, be able to distinguish between them in anything more than varying degrees of approximation.