Secondly, man sins against nature when he goes against his generic nature, that is to say, his animal nature. Now, it is evident that, in accord with natural order, the union of the sexes among animals is ordered towards conception. From this it follows that every sexual intercourse that cannot lead to conception is opposed to man's animal nature.
Thomas AquinasWe are like children, who stand in need of masters to enlighten us and direct us; God has provided for this, by appointing his angels to be our teachers and guides.
Thomas AquinasHe who is not angry when there is just cause for anger is immoral. Why? Because anger looks to the good of justice. And if you can live amid injustice without anger, you are immoral as well as unjust.
Thomas AquinasIt [covetousness] is a sin against God, just as all mortal sins, in as much as man condemns things eternal for the sake of temporal things.
Thomas Aquinas