Likutei Moharan 156:1/ליקוטי מוהר"ן קנ״ו:א׳

1“Lev Tahor (A pure heart) create for me, O Lord, [and an upright spirit renew within me].” (Psalms 51:12)

2That which a person speaks privately with his Master is an aspect of ruach hakodesh (holy spirit). King David, of blessed memory, whose virtue was very great, based the Book of Psalms on this.

3Likewise, [for] each person, commensurate with his level, this is an aspect of holy spirit. As it is written (Psalms 27:8), “To You my heart said”; which Rashi explains: “To You”—for Your sake and at Your charge my heart said to me. Everything the heart says <to a person, to do good or to speak good words before God,> is literally the word of God. This is an aspect of holy spirit.

4Aperson must always originate, each time requesting with original supplications and words of appeasement. To merit to this, he needs a pure heart. A pure heart is the result of his heart’s inspiration and flaming for God. This purifies the heart. As opposed to his being inspired and aflame for a sin or an evil desire, God forbid, from which his heart becomes impure—as opposed to this, he should inspire and enflame his heart for God. {For through the warmth of the heart one dispels the spirit of impurity. This is because it is the nature of fire to dispel air, as is clear to those knowledgeable of the elements. This is the reason a gun shoots when the gunpowder placed at [the hammer of the gun] is lit. From the force of the fire, the air is expelled, and it is this air that propels whatever was loaded in the gun. There are also other things which clarify this. Yet the general principle is that fire dispels air. As a result, the warmth of a heart which is aflame for God dispels the spirits, which are air. That is, it dispels the spirit of impurity, and through this the heart is purified.}

5Through this, his heart will be purified. As it is written (Numbers 31:23), “Anything that was used over fire, must go through fire <and be purified>.” And when he has purity of heart, he then merits to each time speak new words—an aspect of holy spirit.

6This is the meaning [of the opening verse]: A pure heart create for me, O Lord—

7Then: an upright spirit renew within me—He will merit to originate new words, which are an aspect of holy spirit, as mentioned.