Because the virtues also resist those temptations which lead to the sins that are contrary to the virtues; for everything naturally resists its contrary: which is especially clear with regard to charity, of which it is written (Cant.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic virtue writing
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“Others observing that Gregory says (Moral. ii, 26) that "the gift of the Holy Ghost, by coming into the soul endows it with prudence, temperance, justice, and fortitude, and at the same time strengthens it against every kind of temptation by His sevenfold gift," said that the virtues are given us that we may do good works, and the gifts, that we may resist temptation. But neither is this distinction sufficient. Because the virtues also resist those temptations which lead to the sins that are contrary to the virtues; for everything naturally resists its contrary: which is especially clear with regard to charity, of which it is written (Cant. 8:7) : "Many waters cannot quench charity."
Others again, seeing that these gifts are set down in Holy Writ as having been in Christ, according to Isa. 11:2, 3, said that the virtues are given simply that we may do good works, but the gifts, in order to conform us to Christ, chiefly with regard to His Passion, for it was then that these gifts shone with the greatest splendor.” source