Evil denotes the lack of good. Not every absence of good is an evil, for absence may be taken either in a purely negative or in a privative sense. Mere negation does not display the character of evil, otherwise nonexistents would be evil and moreover, a thing would be evil for not possessing the goodness of something else, which would mean that man is bad for not having the strength of a lion or the speed of a wild goat. But what is evil is privation; in this sense blindness means the privation of sight.
Thomas Aquinas (c. 1225–1274), Italian philosopher, theologian. trans. by Thomas Gilby, St. Thomas Aquinas: Philosophical Texts, Oxford University Press, p. 167 (1951). Summa Theologiae, I, q. 18, art. 3 (c. 1077-1078).