In addition to the general power of the mind to control the heart, the Godly soul has an advantage in its struggle to overpower desires originating in the animal soul.
The nature of that additional advantage is expressed in the quote from Ecclesiastes (2:13), "I then saw that there is an advantage of light over darkness," which compares the superiority of wisdom over folly (that is, of good over evil) to the superiority of material light over darkness.
The advantage of light over darkness is not merely qualitative, in the sense that light is better than darkness, nicer, more useful, and more pleasant, but “that a little physical light banishes a great deal of darkness, which is therewith inevitably superseded as a matter of course and necessity.”
Light and darkness do not combat each other as equal contestants.
Rather, light possesses an innate superiority over darkness–the superiority of positivity over negativity.
Light need not battle darkness, for with its very appearance, it repels not only an equal amount of darkness but a lot of darkness.
When two equally positive substances are brought in contest, such as when a solid is immersed in water, the volume of water displaced exactly equals the volume of the solid immersed.
But when one takes a small candle into a dark room, the volume of darkness displaced greatly exceeds the volume of the flame of the candle.
–Rabbi Adin Steinsaltz
From Opening the Tanya, p. 297, by Rabbi Adin Steinsaltz