What Sartre understands by freedom is the unique ability of human beings to rise above their given condition. Plants, rocks, and animals don't have this ability, only humans. Contrary to what determinists believe, man has the freedom to be something other than he is. Unlike animals, for example, he will always be more than just the sum of his biological parts.
As an avowed atheist, Sartre doesn't believe in the existence of the soul; but he does believe that we as humans have something special that distinguishes us from animals. This is the capacity to transcend ourselves, to project ourselves into the future by way of projects and plans. Animals cannot do this; they live in a perpetual present. But human beings live their lives forward, and they can only do this because they possess freedom.
All too often, however, we choose to pretend that we're unable to exercise our capacity for freedom. Instead, we live lives of mind-numbing inauthenticity, kidding ourselves that we have no choice...
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