53 Hence we must watch particularly over the movements of our passions and of our senses, and so control them by voluntary discipline in our lives and by bodily mortification that we render them obedient to right reason and God's law: "And they that are Christ's have crucified their flesh, with its vices and concupiscences." The Apostle of the Gentiles says this about himself: "But I chastise my body, and bring it into subjection: lest perhaps, when I have preached to others, I myself should become a castaway." All holy men and women have most carefully guarded the movements of their senses and their passions, and at times have very harshly crushed them, in keeping with the teaching of the Divine Master: "But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart. And if thy right eye scandalize thee, pluck it out and cast it from thee. For it is expedient for thee that one of thy members should perish, rather than that thy whole body be cast into hell." It is abundantly clear that with this warning Our Savior demands of us above all that we never consent to any sin, even internally, and that we steadfastly remove far from us anything that can even slightly tarnish the beautiful virtue of purity. In this matter no diligence, no severity can be considered exaggerated. If ill health or other reasons do not allow one heavier corporal austerities, yet they never free one from vigilance and internal self-control.
Source: Sacra Virginitas (Vatican.va)