How we cite our quotes: Citations follow this format: (Chapter.Paragraph)
Quote #1
He looked as handsome and as lively as ever, and was talking with interest to a fashionable and pleasant-looking young woman, who leant on his arm, and whom Catherine immediately guessed to be his sister; thus unthinkingly throwing away a fair opportunity of considering him lost for her for ever, by being married already. But guided only by what was simple and probable, it had never entered her head that Mr. Tilney could be married. (8.3)
Catherine actually avoids folly here, by using common sense to not make a faulty assumption. While Catherine makes a lot of foolish assumptions, it's important to note that she does have common sense to draw upon.
Quote #2
Catherine's expectations of pleasure from her visit in Milsom-street were so very high, that disappointment was inevitable; and accordingly [...] she found, on her return, without spending many hours in the examination of her feelings, that she had gone to her appointment preparing for happiness which it had not afforded. (16.1)
Overly high or foolish expectations are a running theme throughout this text. Catherine here sets herself up for a fall by having overly high, and romantic, expectations for her visit to the Tilneys' house in Bath.
Quote #3
"Nay, my sweetest Catherine, this is being quite absurd! Modesty, and all that, is very well in its way, but really a little common honesty is sometimes quite as becoming. I have no idea of being so overstrained! It is fishing for compliments. His attentions were such as a child must have noticed [....] So it is in vain to affect ignorance." (18.11)
This speech is highly ironic coming from the often phony Isabella. She is berating, or scolding, Catherine for the folly of ignorance, or pretended ignorance, regarding John's "obvious" marriage proposal.