Hmmm, St. Paul might disagree with your statement.
Try for example Romans 2:13-15 or Romans 7:15-19.
That was poorly worded. Man is capable of distinguishing good and evil. But where Aquinas believes that man can sit and independently make a choice between doing good or doing evil, I would say (and Paul would agree) that regardless of whether we know something to be wrong, we tend to do the very things that we don’t want to do.