Jean-Paul Sartre
Thought

Sartre's primary idea is that people, as humans, are "condemned to be free". He explained, "This may seem paradoxical because condemnation is normally an external judgment which constitutes the conclusion of a judgment. Here, it is not the human who has chosen to be like this. There is a contingency of human existence. It is a condemnation of their being. Their being is not determined, so it is up to everyone to create their own existence, for which they are then responsible. They cannot not be free, there is a form of necessity for freedom, which can never be given up."

This theory relies upon his position that there is no creator, and is illustrated using the example of the paper cutter. Sartre says that if one considered a paper cutter, one would assume that the creator would have had a plan for it: an essence. Sartre said that human beings have no essence before their existence because there is no Creator. Thus: "existence precedes essence". This forms the basis for his assertion that because one cannot explain one's own actions and behavior by referring to any specific human nature, they are necessarily fully responsible for those actions. "We are left alone, without excuse." "We can act without being determined by our past which is always separated from us."

Sartre maintained that the concepts of authenticity and individuality have to be earned but not learned. We need to experience "death consciousness" so as to wake up ourselves as to what is really important; the authentic in our lives which is life experience, not knowledge. Death draws the final point when we as beings cease to live for ourselves and permanently become objects that exist only for the outside world. In this way death emphasizes the burden of our free, individual existence. "We can oppose authenticity to an inauthentic way of being. Authenticity consists in experiencing the indeterminate character of existence in anguish. It is also to know how to face it by giving meaning to our actions and by recognizing ourselves as the author of this meaning. On the other hand, an inauthentic way of being consists in running away, in lying to oneself in order to escape this anguish and the responsibility for one's own existence."

While Sartre had been influenced by Heidegger, the publication of Being and Nothingness did mark a split in their perspectives, with Heidegger remarking in Letter on Humanism:

Existentialism says existence precedes essence. In this statement he is taking existentia and essentia according to their metaphysical meaning, which, from Plato's time on, has said that essentia precedes existentia. Sartre reverses this statement. But the reversal of a metaphysical statement remains a metaphysical statement. With it, he stays with metaphysics, in oblivion of the truth of Being.

Herbert Marcuse also had issues with Sartre's metaphysical interpretation of human existence in Being and Nothingness and suggested the work projected anxiety and meaninglessness onto the nature of existence itself:

Insofar as Existentialism is a philosophical doctrine, it remains an idealistic doctrine: it hypostatizes specific historical conditions of human existence into ontological and metaphysical characteristics. Existentialism thus becomes part of the very ideology which it attacks, and its radicalism is illusory.

Sartre also took inspiration from phenomenological epistemology, explained by Franz Adler in this way: "Man chooses and makes himself by acting. Any action implies the judgment that he is right under the circumstances not only for the actor, but also for everybody else in similar circumstances." Also important is Sartre's analysis of psychological concepts, including his suggestion that consciousness exists as something other than itself, and that the conscious awareness of things is not limited to their knowledge: for Sartre intentionality applies to the emotions as well as to cognitions, to desires as well as to perceptions. "When an external object is perceived, consciousness is also conscious of itself, even if consciousness is not its own object: it is a non-positional consciousness of itself." However his critique of psychoanalysis, particularly of Freud has faced some counter-critique. Richard Wollheim and Thomas Baldwin argued that Sartre's attempt to show that Sigmund Freud 's theory of the unconscious is mistaken was based on a misinterpretation of Freud.
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