" 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 (l’en-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. "
"A central question in Kierkegaard’s writings is how the individual human being can come to terms with their own existence, for it is that existence which is the most important thing in every person’s life. Unfortunately, we are as if adrift in a infinite sea of possible modes of living with no secure anchor that reason informs us will provide certainty and confidence.
This produces despair and anguish, but in the midst of our “metaphysical sickness” we will face a “crisis,” a crisis which reason and rationality cannot decide.
We are forced to reach a decision anyway and to make a commitment, but only after making what Kierkegaard called a “leap of faith” — a leap that is preceded by an awareness of our own freedom and the fact that we might choose wrongly, but nevertheless we must make a choice if we are to truly live.
Those who have developed the Christian themes of Kierkegaard’s existentialism explicitly focus upon the idea that the leap of faith we make must be one which causes us to surrender ourselves totally to God rather than to insist on a continued reliance upon our own reason. It is, then, a focus upon the triumph of faith over philosophy or intellect. "