To Florus 4.93

Therefore, if sin is natural, it is not voluntary; if it is voluntary, it is not inborn. These two definitions are as opposed to one another as necessity and will, whose affirmation arises from the denial of the other. Just as “voluntary” means nothing other than “not compelled,” so “compelled” means nothing other than “not voluntary.” These two therefore cannot exist, or as it were live, together. Their nature is such that one lives only when the other is extinguished.