Good and evil are essential differences of the act of the will. For good and evil pertain essentially to the will; just as truth and falsehood pertain to the reason, the act of which is distinguished essentially by the difference of truth and falsehood (according as we say that an opinion is true or false.) Consequently, good and evil volition are acts differing in species.
Thomas AquinasSloth is sluggishness of the mind which neglects to begin good...it is evil in its effect, if it so oppresses man as to draw him away entirely from good deeds.
Thomas Aquinas