In George Bernard Shaw's Candida, the eponymous heroine has to decide between two men who love her. Her husband, James Morrell, is a popular clergyman who owes most of his success to Candida. Eugene Marchbanks, a young poet, wants to rescue her from what he considers an insupportably dull life with Morrell. Both men idealize Candida but in slightly different ways. Marchbanks views her as a goddess who is above all petty human concerns. Morrell thinks she is angelic but tends to take her for granted.
It is immediately after Morrell has expressed complete confidence in Candida's goodness and purity that she tells him: "You understand nothing," since she is at that moment considering leaving him. This lack of understanding on Morrell's part is in contrast to Marchbanks who, she has just remarked, understands both of them all too well. Morrell is wounded by her comment and repeats it to her later: "He is the poet, who sees everything; and I am the poor parson, who understands nothing."