In Act IV, Rev. John Hale is talking to Mrs. Elizbeth Proctor and is telling her what he has learned from his experiences with the Salem trials. The main philosophical insight he has is that there is nothing so precious as life. He says that there is no principle that can possibly justify the taking of a person's life.
In addition, Hale says that God does not care about truth nearly as much as He cares about life. Because of these insights, he pleads with Elizabeth to get John to lie to save his life.