Throughout Persuasion, Anne Elliot observes just how irrational others are around her, regardless of how rational the characters think they are. We especially see rationality, or irrationality, played out in Lady Russell's opinions of whom Anne should and should not marry. Anne admires Lady Russell as a very wise friend and good counselor, even though she repeatedly proves herself to be prejudicial. It was Lady Russell's prejudicial perspectives that persuaded Anne to reject Captain Wentworth in the past, just as she is trying to persuade Anne in the present. However, Lady Russell rationalizes her prejudicial beliefs on her analysis of a person's character, showing us that rationality, or lack of it, is a major part of the persuasion theme, amplifying the theme by showing how others try to persuade based on rationality. Instead, Lady Russell's beliefs are proved to be ill-founded, or irrational.
Lady Russell judged Captain Wentworth to be a poor match for Anne, not just because he had no wealth or wealthy connections, but because, even though he had already earned a lot in his Navy career, he had also already spent it. She believed it was a sign that he was impetuous and not to be trusted, as we see in the line, "[Captain Wentworth] had been lucky in his profession; but spending freely, what had come freely, had realized nothing," meaning saved nothing, and again later in the line, "[Lady Russell] had been too quick in suspecting [his character traits] to indicate a character of dangerous impetuosity" (Ch. 4; Ch.24). However, now that Captain Wentworth has returned with a substantial fortune and a great deal of praise and admiration, she is forced to realize that she judged him incorrectly, showing us that what she felt was her rationalization for persuading Anne not to marry him was actually very incorrect, even prejudiced and irrational.
In addition, Lady Russell also judged Mr. Elliot, Anne's cousin, to be an excellent match for her. He was believed to still be wealthy and was also found to have excellent manners, which she felt showed he had an excellent character. His manners are seen when he reconciles himself with Sir Walter Elliot, whom he had offended by not courting Elizabeth and marrying someone else. However, Mr. Elliot is now very open and friendly with Sir Walter and has apologized for the past, so much so that Sir Walter has readily forgiven him. Even Lady Russell is impressed with Mr. Elliot's manners. However, Anne is suspicious. She believes there must be an ulterior motive for Mr. Elliot wanting to re-establish himself in the Elliot family. Later it is discovered that Elliot has only re-established connections with the Elliot family and begun courting Anne because he is determined to keep his Kellynch Hall inheritance and become the next Sir Elliot. However, Sir Walter is presently being pursued by the designing Mrs. Clay, and should she marry Sir Walter, they could produce the next heir together, rather than it being passed down to Mr. Elliot. Therefore Mr. Elliot's design in courting Anne is to both secure Anne's fortune while allowing himself to get close enough to Mrs. Clay that he can intervene in her schemes. Not only is Mr. Elliot proved to be conniving, he is also proved to be a "disingenuous, artificial, worldly man" (Ch. 21). Therefore, Lady Russell is proved to be wrong about Mr. Elliot as well, showing once again that her rationalization for her persuasion is based on prejudice.