Humani Generis §2

² It is not surprising that such discord and error should always have existed outside the fold of Christ. For though, absolutely speaking, human reason by its own natural force and light can arrive at a true and certain knowledge of the one personal God, Who by His providence watches over and governs the world, and also of the natural law, which the Creator has written in our hearts, still there are not a few obstacles to prevent reason from making efficient and fruitful use of its natural ability. The truths that have to do with God and the relations between God and men, completely surpass the sensible order and demand self-surrender and self-abnegation in order to be put into practice and to influence practical life. Now the human intellect, in gaining the knowledge of such truths is hampered both by the activity of the senses and the imagination, and by evil passions arising from original sin. Hence men easily persuade themselves in such matters that what they do not wish to believe is false or at least doubtful.