Clear ideas about cooperation with evil have never been more urgently needed than in the modern world. Nonetheless, confusion still surrounds what has been called the most difficult problem in moral theology. Evaluating the responsibility of a cooperating agent requires understanding the nature of moral evil, the definition of an act's object, and the relevance of both circumstances and consequences. The author presents the history of the issue from the 17th to the 20th century by following the debate about the Servant and the Ladder, a proposition condemned as laxist by Bl Pope Innocent XI....