One of the greatest evils that the sin of Adam has produced in us, is that darkening of our reason by means of the passions which cloud our mind. Oh, how miserable is that soul that allows itself to be ruled by any passion! Passion, is as it were, a vapor, a veil which prevents us from seeing the truth. How can he fly from evil, who does not know what is evil? Besides, this obscurity increases in proportion as our sins increase.
--St. Alphonsus Liguori, The Holy Eucharist