Crucially, Kierkegaard recognises that anxiety, angst, anguish, or dread (whichever term you prefer) is central to the human condition as it is lived and is suffered by every human being. To understand the true nature of anxiety is therefore to understand a great deal about being human.

Firstly, although it is certainly related to fear in various ways, anxiety must be clearly distinguished from fear. In The Concept of Anxiety Kierkegaard argues that fear is a person's concern about what threatens him from outside - from a myriad threats to life, limb, livelihood and happiness over which he has limited control. Anxiety, on the other hand, is a person's concern about what, so to speak, threatens him from inside, from within his own consciousness. An anxious person is concerned about what he might choose to do given his freedom to choose. He is troubled by his own freedom and spontaneity; by the awareness that there is nothing whatsoever preventing him from choosing to perform a foolish, destructive or disreputable act at any moment, other than his choice not to perform it. "Hence," says Kierkegaard, "anxiety is the dizziness of freedom" (p.61). To be anxious is to be bewildered by one's own freedom; to be worried and disturbed by the realisation that one always has many options in any situation and must continually choose one option or another. Not choosing is not an option because choosing not to choose, or choosing to do nothing, is still a choice.

This dizziness of freedom is most clearly manifested in the sensation of vertigo. Kierkegaard takes the example of a man standing on the edge of a tall building or cliff. The man fears he might fall over the edge, that the safety rail or the ground might give way, that someone might push him off, and so on. Greater than his fear of falling, however, is his anxiety that he is free to jump if he decides to - that his not jumping is an ongoing choice which he might abandon at any moment in favour of jumping. He experiences this anxiety, the threat of his own freedom, as vertigo, an overwhelming giddiness. The drop obsesses him, the void seems to beckon him down; but really it is his own freedom that beckons to him - the very fact that he can always choose to go down the quick way. Vertigo is dread of this alarming and persistent possibility, and all our alarming possibilities produce in us a psychological state akin to vertigo. That is to say, what a person overlooking a sheer drop dreads is not the possible inadequacy of the physical guard rail, but that he ultimately lacks a psychological guard rail to prevent him from choosing to climb over and plunge to his death. If it appears on the face of it that his dread is of the void itself, this is because his vivid awareness of the void immediately forces him to confront his own possibilities, his own dreadful existential freedom. The void is the occasion of his dread, but not its source.

Interestingly, if the man fancies he has a fixed psychological guard rail that prevents him from choosing to jump, then he is deluding himself - in existentialist jargon he is resorting to bad faith - because whatever psychological barrier he possesses is merely a flimsy construct consisting of nothing more than the choice not to jump, a choice he is free to replace at any moment with a self-destructive decision. His anxiety is precisely his awareness of the ease with which he can spontaneously overturn the self-determinations that he wishes would permanently fix, define, preserve, and protect him.