by Reb Aharon Rubin @ Mystical Paths
(Excerpted from the book Eye to the Infinite)
The two kinds of imagery (יצר מחשבות) produced by the two Yetsers means that man operates on two levels. The Yetser HoRo, concerned with survival, protecting and projecting the self in this physical world, will produce imagery inclined to be self-centered, thus producing behaviours prone to be narcissistic and even immoral. The Yetser HaTov, on the other hand, is concerned with the soul. It is selfless; it thus produces tends to the transcendental, the divine, and is therefore the source of our ethical and spiritual, yearnings and convictions. Which of the two yetsers influences and determines our thoughts, speech or deeds, is dependent upon the focus and emphasis of the conscious mind.
Therein lies the miracle of free-choice. When the focus is external, contingent upon peripheral factors such as honour and gratification, the Yetser HoRo will be involved; if the motivation is unselfish and internally directed, the focus G-d related, then the Yetser HaTov will be involved. This is a continuous struggle within the human psyche, a mêlée for the attention of the conscious mind in every trial and incident within human experience: to focus on the physical, the self, or on the inner, Theocentric aspect?
Since the Yetser HaTov is closest to the Neshomoh and most directly associates with its holy aspirations and the Neshomoh, in its interaction with the physical, is closest to thought (see Part One, Chapter One), the Yetser HaTov most readily associates with quiet reflection and contemplation. Therefore, by increasing meditative patterns of thinking, you may subdue the Yetser HoRo in its race to the crass and immoral. As the Yetser HaTov works contemplatively, the sages advise us to avoid anger, to think things over and not act hastily, since slower, more meditative, brain-action automatically associates with the Yetser HaTov. In a similar vein, Rabbi Avrohom, son of Maimonides, writes:
The Yetser Tov is the intellect [Da’as] and the Yetser HoRo is the bodily desires. One should always ascertain that the intellect rules over bodily pleasures. This should be uppermost in his mind. If [the power of] thought is enough to subdue his desires, that is the best. If not, he should add to thought the chanting and articulation of verses and words that will serve to subdue his desire and [thus] he should stop his thoughts from thinking other thoughts ...”
Because both the Yetser HaTov and the Yetser HoRo relate to the conscious and the subconscious minds, and both yetsers feed off the Neshomoh’s energy, the greater the Neshomoh, the greater the two yetsers’ strength. Moreover, the greater the Neshomoh, the greater the vessel [Yetser HoRo] needed to contain it. So, although the ego of the spiritually advanced person serves his soul, there is always a greater danger of the bigger shell taking control, resulting in delusions of grandeur or paranoia.
 See Genesis 6:5.
 See Tanya, Chapter 1.
 Rabbi Chaim Vital, Sha'arei Qedushoh, ibid.
 Brochos 17a, 29b, Pesochim 113b.
 Sefer Chassidim 137.
 Zohar Chodosh, 21.
 Essay on the Aggadoth of Chazal. See introductions to Ein Yaaqov and Brochos 5a.