Sartre rejects the existence of God in a tidy argument. If a sovereign God existed, then persons would not be free agents. Persons are free agents, however. So a sovereign God does not exist. Further, if persons are not divinely created beings, then there was no plan and no blueprint for what they are intended to be.

“Man first of all exists, encounters himself, surges up in the world — and defines himself afterward,” Sartre says. He faults previous atheists for supposing they could remove the concept of God from their systems and still go on talking about human nature and objective values. Instead, “There is no human nature, because there is no God to have a conception of it.”

Without a human nature to appeal to, you are condemned to be free.

Sartre says you do not have freedom, you are freedom. Freedom is not one property among many, but is intrinsic to the sort of beings you are, for at each moment of your existence you are creating yourself anew. “We are condemned to be free,” he says, because freedom is a great human burden. For with freedom comes responsibility for your life. Blame cannot be placed on your environment or on God.

You are born into a situation. Those features of your existence that you cannot change Sartre calls “facticity.” So, you were born into this environment, not another, born to these parents, not some others. Therefore, you might look at these facts as impositions on your freedom caused by past events over which you had no control. But these facts don't have a meaning until you assign them a meaning. Your true freedom comes to the fore in the ways in which you respond to your facticity.

If you were born into affluence, you can continue to pursue that life as you grow older, or you can shun that life and join the priesthood, or you can fight for the rights of the poor. So it is with gender. Gender is a biological fact, but this bit of facticity tells nothing about what it means to be this or that gender. Hence, scientific facts tell you very little about how to live your life. You still have significant freedom in determining the outcome of your life.

Sartre quotes Dostoyevsky's pronouncement “If God does not exist, every thing would be permitted.” There are no objective values or religious commandments to appeal to. “We have neither behind us, no before us in a luminous realm of values, any means of justification or excuse,” Sartre says. “We are left alone, without excuse.”

He tells the story of a poor, orphaned man who had a disastrous love affair and was denied a military career because he failed an exam. He believes his terrible situation is a sign he should serve God and becomes a Jesuit priest. Sartre says this is the meaning that this individual chose to assign to these experiences. However, he could just as easily decide that they mean he should become a revolutionary. There is no moral almanac to tell the individual what to do. The gist of Sartre's philosophy is that individuals give these facts meaning by deciding how to act.