Aristotle situates freedom in nature and slavery in reason. His concept of freedom is inherently connected with the indeterminist belief in a double impulse of the body.The deterministic conception of nature – introduced during Enlightenment – has brought a reversal of this relation: nature is slavery, reason is freedom. The double impulse is reduced to one impulse only. The different conceptions of nature result in different conceptions of law.During the second half of the twentieth century the deterministic conception is rejected. One of the few authors who tried to envisage the ontological consequences of this rejection was Popper in his later work.This contribution will show how Popper tries to save the idea that reason is freedom within the context of the new indeterminist view on nature. It will compare Popper's attempt with the view of Aristotle.