Originated by Jean-Paul Sartre, the phrase existence precedes essence has come to be a classic, even defining, formulation of the heart of existentialist philosophy. Its an idea which turns traditional metaphysics on its head because throughout Western philosophy, it was always assumed that the essence or nature of a thing is more fundamental and eternal than its mere existence. Thus, if you want to understand a thing, what you must do is learn more about its essence.
It should be understood that Sartre does not apply this principle universally, but only to humanity. Sartre argued that there were essentially two kinds of being. The first is being-in-itself (len-soi), which is characterized as fixed, complete, and having absolutely no reason for its being it just is. This describes the world of external objects. The second is being-for-itself (le pour-soi), which is characterized as dependent upon the former for its existence. It has no absolute, fixed, eternal nature and describes the state of humanity.
Sartre, like Husserl, argued that it is an error to treat human beings in the same way we treat external objects. When we consider, for example, a hammer, we can understand its nature by listing its properties and examining the purpose for which it was created. Hammers are made by people for certain reasons in a sense, the essence or nature of a hammer exists in the mind of the creator before the actual hammer exists in the world. Thus, one can say that when it comes to things like hammers, essence precedes existence.
But is the same true of human beings? Traditionally this was assumed to be the case because people believed that humans were created by God. According to traditional Christian mythology, humanity was created by God through a deliberate act of will and with specific ideas in mind God knew what was to be made before humans ever existed. Thus, in the context of Christianity, humans are like hammers because the essence (nature, characteristics) of humanity existed in the eternal mind of God before any actual humans existed in the world.
Even many atheists retained this basic premise despite the fact that they had dispensed with the accompanying premise of God. They assumed that human beings possessed some special human nature which constrained what a person could or could not be basically, that they all possessed some essence that preceded their existence.
Sartre, however, goes a step further and rejects this idea, arguing that it was necessary for anyone who was going to take atheism seriously. It isnt enough to simply abandon the concept of God, one has to also abandon any concepts which derived from and were dependent upon God no matter how comfortable and familiar they might have become over the centuries.
Sartre draws two important conclusions from this. First, he argues that there is no given human nature common to everyone because there is no God to give it in the first place. Human beings exist, that much is clear, but it is only after they exist that some essence that can be called human may develop. Human beings must develop, define, and decide what their nature will be through an engagement with themselves, their society, and the natural world around them.
Second, Sartre argues that because the nature of every human being is dependent upon that person, this radical freedom is accompanied by an equally radical responsibility. No one can simply say it was in my nature as an excuse for some behavior of theirs. Whatever a person is or does is wholly dependent upon their own choices and commitments there is nothing else to fall back upon. People have no one to blame (or praise) but themselves.
Just at this moment of extreme individualism, however, Sartre steps back and reminds us that we arent isolated individuals but rather members of communities and of the human race. There may not be a universal human nature, but there is certainly a common human condition we are all in this together, we all living in human society, and we are all faced with the same sorts of decisions.
Whenever we make choices about what to do and make commitments about how to live our lives, we are also making the statement that this behavior and this commitment is something that is of value and important to human beings in other words, despite the fact that there is no objective authority telling us how to behave, this is still something that others should choose as well.
Thus, our choices not only affect ourselves, they also affect others. This means, in turn, that we are not only responsible for ourselves but also bear some responsibility for others for what they choose and what they do. It would be an act of self-deception to make a choice and then at the same time wish that others would not make the same choice. Accepting some responsibility for others following our lead is the only alternative.