One of existentialism's main principles is that humans are free, self-conscious beings. This means that each human must define who he is while accepting the responsibility that accompanies freedom. Each human is responsible for his actions and decisions; regardless of external influence, he is fundamentally alone in an uncertain world.
Because humans are free agents, and there is no single way to navigate through life, it is impossible to reliably predict other agents' behavior. Sartre referred to this inescapable uncertainty as despair.
Nietzsche recognized that there is no intrinsic meaning to life; however, he believed that people can create meaning and value depending on the way they live their lives. Satre continued this line of reasoning to arrive at another of existentialism's principles: since there is no objective account of what it means to be human, each human decides his own meaning through existing. Human beings are not fixed entities; rather, they decide what they become.
Alienation is also central to existentialism. Alienation refers to the fact that, while humans can give the world meaning through their actions, the world is not brought into being through human actions. Every human is conscious of the world's otherness at various times; for example, the self-consciousness he feels when he realizes someone is watching him. Every human may choose his own actions, but he is also merely part of the world for other people.
Another principle is that of authenticity, which is concerned with self-making. An existentialist considers an authentic life to be one in which a person chooses his actions based on his values and commits to them. An inauthentic person carries out his actions because that is what is assumed of him, and he is much more passive.