...no one before Descartes had stressed the connection between free will and negativity. No one had shown that freedom does not come from man as he is, as a fullness of existence among other fulnnesses in a world without lacunae, but rather from man as he is not, from man as a finite, limited being. However, this freedom can in no way be creative, since it is nothing. It has no power to produce ideas, for an idea is a reality, that is, it possesses a certain being that I cannot confer upon it. In addition, Descartes himself limited its scope, since, according to him, when being finally appears...we cannot refuse it our adherence. WE can thus see that he did not push his theory of negativity to the limit: 'Since truth consists in being and falsehood in non-being only.' Man's power of refusal lies only in his refusing the false, in short, in saying no to non-being. If we are able to withhold our assent to the works of the Evil Spirit, it is not because they are true or false -- they have at least, insofar as they are our conceptions, a minimum of being -- but insofar as they are not, that is, insofar as they relate falsely to objects that do not exist...Thus, Descartes constantly wavers between the identification of freedom with the negativity or negation of being -- which would be the freedom of indifference -- and the conception of free will as a simple negation of negation. In short, he failed to conceive negativity as productive.
Sartre, in Cartesian Freedom