What does Sartre mean with that existence precedes essence and how is it related to the earlier existential philosophers' thoughts?
Existential philosophers like Kirkegaard, Heideggers and Friedrich Nietzsche?
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Existence precedes essence means human beings are not defined by an a priori nature. Rather, what's fundamental about us is that we exist. This stands in contrast to traditional philosophies and theologies which claimed people had an essence (e.g. man is a reasoning animal, man has a divine spark, etc...).
Bare existence lacks meaning, as such we are faced with dread, awe, and to believe in something greater requires a leap of faith (Kierkegaard), for we don't have the comfort of "knowing" what to do. Additionally, the lack of meaning invites nihilism, atheism, iconoclasm and of course freedom and its implications on morality, and this is where Nietzsche comes in.
Furthermore, bare existence is about consciousness, and as such about being in the world, phenomenology, how we relate to time. Heidegger figures prominently here.
What Sartre means is that first we exist as free beings that have absolute freedom to do anything, and then we can retrospectively assign essence to ourselves.
The key concept here is freedom. Sartre was 100% for free will, and he rejected any kind of determinist argument (regarding human agency, not nature) as 'determinist mythology'. If we have an essence, it would mean that we are determined by that essence.
Believing that essence precedes existence leads to 'bad faith', which you can clearly observe in people who associate too much of their being with their job. They start to act unnaturally, and it often comes off as weird, annoying and pretentious. For example, I am a software developer, most people in this field are highly intelligent and quite nice, but very frequently they behave in this autistic manner that really annoys me. Usually a guy might tell you to 'refactor the API endpoint to fix a bug', even though he knows perfectly well that you have no idea what the **** he is talking about.
Bad faith is fake, it is inauthentic, but is is a natural human tendency because freedom makes us feel anxious, think about the last time when you were at the grocery store and could not decide between pistachio and strawberry ice creams.
Kierkegaard also thought that freedom makes us anxious. For Kierkegaard, anxiety is a part of our existential condition and it is not directed at a particular object, unlike fear.
Heidegger agreed with Schelling that we have free will, and he had several seminars on Schelling.
Nietzsche did not explicitly address the free will / determinism argument. He thought we should create our own standards for greatness and be masters of our fate, he was a free spirit kind of thinker.
It's part of a sea change of philosophy from idealism to materialism. Before Sartre, essence was seen to precede existence. Thus essence was seen as ideal and not material. For examlle, a man's soul. After Sartre, existence was seen to precede essence and thus essence was seen to be also material. Thus soul, was also material. It's where all this blather of neuro-philosophy and its like derives from.
As @CriglCragl doubted the substance of my reply above, I'm adding a quote from Wikipedia's Existence Precedes Essence:
The idea originates from a speech by Schelling delivered in December 1841. Søren Kierkegaard was present at this occasion and the idea can be found in Kierkegaard's works in the 19th century, but was explicitly formulated by philosopher Jean-Paul Sartre in the 20th century. The three-word formula originated in his 1945 lecture "Existentialism Is a Humanism", though antecedent notions can be found in Heidegger's Being and Time.
Sarte, as I see it, is saying we're free to define our lives any which way we want to. There's no predefined essence that could, against our will, chart the course of lives. "We're condemned to be free" ~ Sartre. His philosophy seems premised on the existence free will and the nonexistence of nature.