"For R. Huna said: Once a man does wrong and repeats it, it is permitted to him. ‘It is permitted to him’! Can you really think so? — Rather it becomes to him as something permitted."
Talmud Bavli, Kiddushin 40a
"To those who are physically sick, the bitter tastes sweet and the sweet bitter. Some of the sick even desire and crave that which is not fit to eat, such as earth and charcoal, and hate healthful foods, such as bread and meat - all depending on how serious the sickness is.
Similarly, those who are morally ill desire and love bad traits, hate the good path, and are lazy to follow it. Depending on how sick they are, they find it exceedingly burdensome.
Isaiah 5:20 speaks of such people in a like manner: "Woe to those who call the bad good, and the good bad, who take darkness to be light and light to be darkness, who take bitter to be sweet and sweet to be bitter." Concerning them, Proverbs 2:13 states: "Those who leave the upright paths to walk in the ways of darkness."
Rambam, Mishnah Torah, Hilkhot De'ot 2:1
Jacques Barzun, From Dawn To Decadence