Thomas Aquinas, Compendium of Theology

A question worthy of consideration arises at this point. As the term "good" signifies perfect being, so the term "evil" signifies nothing else than privation of perfect being. In its proper acceptation, privation is predicated of that which is fitted by its nature to be possessed, and to be possessed at a certain time and in a certain manner. Evidently, therefore, a thing is called evil if it lacks a perfection it ought to have. Thus if a man lacks the sense of sight, this is an evil for him. But the same lack is not an evil for a stone, for the stone is not equipped by nature to have the faculty of sight.