Man's sin consists principally in what is contrary to right reason. As sickness takes place in the body by reason of a disorder of some humor, so too sin against reason takes place in the soul by reason of a disorder of some circumstance. Hence sometimes a person sins in the matter of fear from the fact that he fears what he ought not to fear, but other times from the fact that he fears when he ought not to fear.
[Thomas Aquinas, Commentary on Aristotle's Nicomachean Ethics, Litzinger, tr., Dumb Ox Books (Notre Dame, IN: 1993) p. 180]
