Sartre is also a leading figure in phenomenology, a branch of philosophy that offered a radical account of the workings of human consciousness. In other words, it is experiencing reality as we experience it with our perceptions, distinguished from the world as it really is.
He studied under Edmund Husserl, the world’s leader in that field. He felt that he found an entirely new way of seeing man’s existence in the world.
To understand Sartre’s view of what phenomenology is, a good starting point is his 1945 public lecture Existentialism is a Humanism, which was later made into a book, he declared his famous proposition that for human beings “Existence precedes essence”, that is the fundamental tenet of Existentialism.
Ancient Greek philosophers Plato and Aristotle believed that every object had within it an essence. The essence of a thing is a specific thing within an object that need to be there for that thing to be considered as whatever it is. If this thing for some reason did no longer have this specific property, it’d lose its identity and would therefore become something else.
Take for instance a knife, if it lacked its blade, it would just be a colourful handle of a sort. In other words, it’d have lost its essential property or the essence of that knife, a quality that is necessary to make it what it is.
This extends to every single object, including human beings. Every human being is born with this essence; thus, essence precedes existence. This is the core philosophy of Essentialism.
This theory remained strong since the time of ancient Greek philosophy right until the 20th century. Here is where Sartre comes in, and asks: what if we are born without an essence? What if when we are born, we are to determine what our essence will be? What if existence precedes essence? In other words, an individual creates himself through what he does, he is what he does. Just as a painter paints on a blank canvas, we invent what will eventually appear on the canvas. In that way, our life is a work of art and every action defines us.
However, the moment when you realise that your existence is not founded upon any past objective facts, that your existence consists of what you’re going to make of it, it becomes a slightly horrifying realisation.