Jean-Paul Sartre
One of the only self-described existentialists, Sartre brought the philosophy of consciousness and free will to the mainstream using plays, novels, volumes, essays, and magazine articles.
Free Will
Phenomenological Ontology
Consciousness
Negation
Sartre's Metaphysics
"Existentialism is the doctrine that existence precedes essence." In other words, from the moment you are born, you choose yourself through action, there is no "human nature" to fall back upon. People are not created for a purpose, they must define their purpose through action. This is the essence of existentialist athiesm: There is no God that created us with a set image in mind, man is free to define himself in whatever way he pleases. Because of realizing this responsibility, there is bound to be anguish, what Kirkeguaard calls the "anguish of Abraham," when he was called by God to slay his son Isaac. The omens in life are not determined, it is up to us to decide how they should fit in with our lives.
Picking up from where we left off from phenomenology, he thought there were three modes of consciousness capable of perceiving an object: perception, imagination, and conception. Sartre uses Husserl's analogy of looking at a cube to relate his idea of perception in Abschattungen. Abschattung is German for "in-profile," we can see, at most, 3 sides of the cube at once. How do we know it is a cube? If viewed from the back, it very well may be hollow, with only 5 sides, but we percieve it to be a cube. Perception, in the Sartrian sense, always carries a kind of promise, a set of inferences.
An unlimited set of inferences actually, as you can continously make guesses in succession concerning an object, and have them proven either correct or incorrect. This is what he means when, in the introduction to Being and Nothingness, where he says to have eradicated all dualisms excepting the infinite and finite. He also says that, for Husserl, "essence is the principle of the series." All this means is that, if you are observing a cube, its essence, i.e., the summation of its infinite probabilities, tells you that it is most likely a cube. According to Husserl, the essence itself can be made a phenomena itself (Lecture II).
His question was, "if the appearance is no longer opposed to being, what about the being of the phenomenon? Is it the same as the phenomena?" Can the being of the phenomenon be realized by its essence?
As for perception, try imagining Husserl's cube. You imagine it as a cube, therefore when you view it (still in Abschattungen), and rotate it, it remains a cube, the promises of "more to come" are always fulfilled. So we can say that the imagination is subjective, being it is our playground. We make the rules. Perceptions are objective in the way that, though they are testable, their essences are not necessarily true. As for conception there is no visualization, therefore no "promise," and no chance of making a mistake. Think of it as conceiving, not imagining , a 6-sided cube, in a mathematical way.
His systematic work, Being and Nothingness, is a 700+ page hybrid of existential and phenomenological thought. In it, he distinguishes things with consciousness (being-for-itself) and objects devoid of will (being-in-itself). What does this mean?
Being-in-itself
...is a deliberate play on Kant's thing-in-itself, the unknowable noumena that met our constituting Ego halfway. Sartre accepts both intentionality of consciousness and constitution. The being-in-itself is like a polygon whose number of sides (manifestations) are infinitely increasing. We can know being-in-itself through 5 of them, and can only infer the rest. That's where the theory of intentionality comes in: the statue does not hide the bronze, it is the bronze. But the bronze is not the statue. We are taking the formless being-in-itself, and applying a form to it (constitution). Being-in-itself simply is, with no real reason for its existence, just a changeless thing contained completely within itself. Sartre says that being-in-itself "is what is is, and is not what it is not."
Being-for-itself
In contrast is the being-for-itself, which "isn't what is is, and is what it isn't." What does this mean? That consciousness has its own non-being, its own negation. These must originate in consciousness (constitution), for being-in-itself is purely affirmative.
All consciousness is consciousness of *something*. This means that all consciousness posits a transcendent object, or that consciousness has no "content." A table does not exist in consciousness, it exists in space.
Sartre's conception of free will faded a bit, as he absorbed more and more Marxist literature. In his Critique of Dialectical Reason, he argued that humans are born with free will, but that it is gradually degraded to fit in with society's demands. He refers to this Marxist determinism as "serialization." In the end, Sartre pretty much refuted existentialism.