 The position Arnauld defended and attributed to Augustine and Aquinas is, however, quite different from what is nowadays referred to as “compatibilism.” The position is connected with Arnauld’s treatment of free will during the last decade of his life, at the time when he was wrestling with Malebranche’s theory of nature and grace. At that time, Arnauld developd the position that the human will is free when and only when it is a “potestas ou facultas ad opposita.” But first, it is necessary to provide some background for the notion of grâce efficace par elle-même.