Sartre's existentialism: existence precedes essence and free will

¶ … Existence proceeds essence

What is meant be the phrase 'existence precedes essence?' How does this phrase relate to the issue of free will, according to Sartre?

The existentialist contention that existence precedes essence turns the conventional assumptions of Western philosophy on its head. For example, the Greek philosopher Plato used to idealize the world of the Forms, a realm which contained the purest essences all things, and stated that such perfection was what all entities were striving to attain. The perfect ideal and the thought preceded the more imperfect execution. This is also seen in human life. For example, "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" (Cline 2009). Christians believed the idea of human life preceded existence -- first God conceptualized the human, then humans came into being.

Sartre rejected such Platonic idealism, and as an atheist who revered human thought, he also rejected the idea that human beings are like hammers. Humans are unique in their capacity, Sartre believed, for premeditated thought. But that thought must be developed, it is not something we are 'given' at birth -- we have the responsibility to craft that sense of autonomy with our own free will.