5 And on this account that, which, the parts that beget being bridled by modesty, is most chiefly and properly to be called Continence, is violated by no transgression, if the higher Continence, concerning which we have been some time speaking, be preserved in the heart. For this reason the Lord, after He had said, “For from the heart go forth evil thoughts,” then went on to add what it is that belongs to evil thoughts, “murders, adulteries,” and the rest. He spoke not of all; but, having named certain by way of instance, He taught that we are to understand others also.
Of which there is no one that can take place, unless an evil thought have gone before, whereby that is prepared within which is done without, and going forth out of the mouth of the heart already defiles the man, although, through no power being granted, it be not done without by means of the members of the body. When therefore a door of Continence has been set in the mouth of the heart, whence go out all that defile the man, if nothing such be permitted to go out thence, there follows a purity, wherein now the conscience may rejoice; although there be not as yet that perfection, wherein Continence shall not strive with vice.
But now, so long as “the flesh lusts against the spirit, and the spirit against the flesh,” it is enough for us not to consent unto the evils which we feel in us. But, when that consent takes place, then there goes out of the mouth of the heart what defiles the man. But when through Continence consent is withheld, the evil of the lust of the flesh, against which the lust of the spirit fights, is not suffered to harm.
Source: On Continence (New Advent)