We can't have full knowledge all at once. We must start by believing; then afterwards we may be led on to master the evidence for ourselves.
Thomas AquinasEvery judgement of conscience, be it right or wrong, be it about things evil in themselves or morally indifferent, is obligatory, in such wise that he who acts against his conscience always sins.
Thomas AquinasEvil 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 aprivative 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