Sartre

Sartre says that the basic premise of existentialism is that for humans, existence precedes essence.  In other words, a person must exist before she has an essence (character, personality). 

Existentialism has been accused of 1) emphasizing all that is despicable about humanity, 2) encouraging inaction and despair, 3) undermining the validity of human existence, and 4) undermining human solidarity by focusing on subjectivity.

In response to (1), Sartre says that existentialist fiction may seem more despicable because the characters in existentialist novels and play are presented as being responsible for their behavior.  In other words, villains are villains because they want to be terrible people (whereas other literature may present them as a victim of circumstance).

In response to (2), Sartre says that the kind of despair important to existentialism is the basis for action.  Despair is just what happens when you have a decision to make and you contemplate which option would be best.  The decision-making process is despair.  Also, according to existentialism, you are defined by your actions.  So someone who does not act is 'nobody'.

In response to (3), Sartre says that all projects have validity insofar as they are chosen by a free consciousness.  Although there may not be an external, eternal source of values, we can validate values in an intersubjective space.

In response to (4), Sartre points out that the 'subjectivity' existentialism is concerned with is an intersubjectivity.  This means that each individual is within a community and each person is constituted by his or her social connections.  There are no meaningful projects without social meanings.  So Sartre says that subjectivity (as intersubjectivity) should be understood not as individual consciuosness but our ability to go beyond individual consciousness.