There is, I believe, in every disposition a tendency to some particular evil – a natural defect, which not even the best education can overcome.
– Jane Austen
Pride and Prejudice, Chapter 11. The very serious Darcy is trying his best to explain himself to Elizabeth, while she verbally plays with and appears to enjoy teasing him. Darcy is referring here to his own flaw, that when he loses his good opinion of someone, that will never change.