Libertarianism is an ethical system. As an ethical system it classifies behaviors into those that are acceptable and those that are not. The only behaviors that libertarianism defines as unacceptable are those that cause conflict. That includes directly causing conflict, e.g. physically attacking someone, as well as indirectly causing conflict, e.g. provoking a fight between two other people.
You could try to list all of the behaviors that libertarianism does not allow and you would get a fairly good sense for what libertarianism is against: theft, murder, fraud, assault, battery, extortion, rape, embezzlement, and so on. Yet, all of these stem from the simple rule mentioned earlier: do not cause conflict. This rule is called the non-aggression principle and it is the guiding star of libertarianism. Though to really understand libertarianism, what is needed is not just examples of aggression but a clear definition of conflict.
When people interact, there are only two possibilities: cooperation and conflict. Cooperation is when people interact and the things they are doing are compatible. If two people dance together, that is cooperation. On the other hand, conflict is when people interact in ways that their actions are not compatible. If one person is going to the grocery store, and another person steals the money that they were going to use to buy food, that is conflict.
One thing to keep in mind is that libertarianism only says not to cause conflict, which is different from being involved in a conflict. If someone attacks you, then you can defend yourself if you want because you are not starting the fight.
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