There are things which man can not control in his life, but he can assume his past and change himself if what he is does not correspond to his scale of values. Man is the only creator of himself and therefore, his own possessor. Which also makes him the only one who is responsible for his life. Subjectivity is to be understood from this perspective However, the concept under discussion is a bit wider. It also refers to the fact that human nature is a limit for the human. People can not go beyond their own nature and capacity to build the world and ultimately themselves. When creating himself, man will obey certain criteria. Naturally, he will want for these criteria to be universal. That is, while man has the power to decide, he will decide not only for himself as an individual, but for all the individuals

Since man is endowed with reason and able to distinguish the good from the bad, it is safe to assume that the choices he makes shall reflect his values. It would be absurd to think that a less valuable option could be chosen instead of a more valuable one, under the circumstances in which man is driven towards the achievement of happiness.

The implications are of fundamental importance. If god does not exist, nor do the conceptions of good and evil. They too are the responsibility of man. Since there is no determinism, then the only conclusion we can reach is that man is free and that freedom is that which defines him and his nature. The main interest of the existentialist philosophy is not to prove that god does not exist, but to analyze the crisis that man undergoes when becoming aware that his only possession is himself and that the only meaning he can find is inside and through himself

It is a humanism because it identifies the demons man needs to fight, such as angst and despair, which derive from the awareness of the objective truth, nonetheless believing that they can be defeated and man can triumph in constructing himself, manifesting his will and his freedom.
Kaufman, Walter. Existentialism from Dostoyevsky to Sartre.(Meridian Publishing Company, 1989 ) (Philip Mairet translator), 12

"Existentialism" in the Stanford Encyclopaedia of philosophy (Available from http://www.libs.uga.edu/ref/turabian.html, accessed March 26, 2009)

Kierkegaards, Soren. Either/or: A fragment of life (Penguin classics, 1992), 127

Sartre, Jean Paul. Existentialism is a humanism ( Yale University Press. 2007), 25

Kaufman, Walter. Existentialism from Dostoyevsky to Sartre.(Meridian Publishing Company, 1989 ) (Philip Mairet translator), 26

idem

Marcel, Gabriel. The philosophy of existentialism (Citadel, 2002), 159

Bibliography:

"Existentialism" in the Stanford Encyclopaedia of philosophy. Available from http://www.libs.uga.edu/ref/turabian.html, accessed March 26, 2009

Kaufman, Walter. Existentialism from Dostoyevsky to Sartre. Meridian Publishing Company, 1989 (Philip Mairet translator)

Kierkegaards, Soren. Either/or: A fragment of life. Penguin classics, 1992

Marcel, Gabriel. The philosophy of existentialism. Citadel, 2002

Sartre, Jean Paul. Existentialism is a humanism. Yale University Press. 2007