Sartre existentialism is deeply sophistic. Man set the standards for all humanity (Sartre 1945, 2). It is believed that God does not exist, and that man is alone. In this way man is condemned to be free (Sartre 1945, 2). Man relies only on his own will, and the reality out there is in the actions of men (Sartre 1945, 3). Defining man according to these premises Sartre says that man is nothing than his own project and exist only in so far as he carries it out (Sartre 1945, 3). The Cartesian cogito ergo sum (‘I think, I am’) is the only starting point, and that gives man the dignity of not being a mere object (Sartre 1945, 4). 
Cogito is also the place where we discover others, we discover an intersubjective world (Sartre 1945, 4). This universality of man is perpetually re-built as everyone acts. Engagement is the way to define man, and the choices he makes are founded either on truth or error (Sartre 1945, 5). Freedom is willed for the sake of freedom and it depends on the freedom of others. 
This is the only universe out there, the universe of human subjectivity (Sartre 1945, 5).

Sartre, Jean-Paul. “Existentialism is a Humanism.” http://www.btinternet.com/~glynhughes/squashed/sartre.htm.

Tags: Sartre, Existence, Knowledge, Truth, Freedom