Whenever he acted out of strong emotion, this was never an emotion he felt. Instead, it was always simply a way out of boredom. Action is taken not for the sake of reaching a goal, but only for the sake of doing something. An intelligent person, the Underground Man insists, can never act sincerely, and insincere action can only be sustained for a short period of time. The result is that the only way to really live is through self-deception.
Action is based on a confusion of primary and secondary causes.
What this means is that when we act, we need some reason for acting. Such a reason can be provided by love, for example, or justice. If one is moved purely by justice, then one can bring oneself to act, but in order to be able to act, one needs to believe that justice is the primary cause for action. The Underground Man claims that such primary causes are never possible because they can always be broken down to other causes. Ultimately, every action is caused by the laws of nature.