Musar do Liczb 35:34

To what may this be compared? To one whose house is full of mud and mire. Even if he will bring into his house the finest golden vessels, it will not be beautified thereby. He must first remove the mud and mire and then bring in the vessels. And so is it in our instance. The Jew has been given the power through his cleaving to Torah and mitzvoth to root in his soul the holiness of the L-rd, as it is written (Bamidbar 35:34): "I, the L-rd, dwell in the midst of Israel." But when is this? When he does not allow the yetzer to reside there through his false ideas or filthy thoughts. And this is the intent of "And you shall not go astray after your hearts and after your eyes, after which you stray." (That is, [this will occur] only if you guard yourselves in the future against your straying). "so that you remember and do all of My mitzvoth, etc." That is, if you are careful not to go astray, as mentioned above, the great result will follow that through the doing of the mitzvoth you will be holy to the L-rd. (As the Men of the Great Assembly have formulated for the blessing over a mitzvah: "who sanctified us with His mitzvoth." ["so that" refers to "and you will be holy" (i.e., "so that, in remembering and doing My mitzvoth, you will be holy to your G-d")]. But if, G-d forbid, you do go astray, the mitzvoth will not avail for your becoming holy. And this is the prophet's intent in (Jeremiah 4:3): "Plow for yourselves a furrow and do not sow upon thorns." Happy is he who reflects upon this. It will be good for him in this world and in the next.

Until now we have spoken about setting to rights the faculty of speech, the first fundamental, the demarcation between "man" and "animal," for which reason Scripture gives it precedence, viz. (Devarim 30:19): "For the thing is very near to you, in your mouth and in your heart [i.e., your thoughts] to do it." And now we shall speak a little about the faculty of thought, which resides in a man's heart, viz.: "and in your heart to do it." Scripture writes (Psalms 24:3-4): "Who shall ascend the mountain of the L-rd, etc.? …The clean of hands [i.e., one who is clean of theft] and the pure of heart." We learn from this that a man's thoughts must be pure and not intermixed with vanity. We say every day: "Make our hearts one, to love and to fear Your name, and let us not be ashamed, forever." This means what it says. It is known that what resides in a Jew's heart always, is faith in the L-rd, the G-d of the heavens and His Torah, this being the essence of holiness [as intimated in Bamidbar 35:39: "I am the L-rd who dwells in the midst of the children of Israel."] For this reason we ask the Blessed L-rd that He make our hearts one, to love and to fear His name, and that there not be intermixed in this another love. For if in his heart, the locus of thought, there also is planted a love for the vanities of the world, in the end, he will be shamed and mortified by it forever. For it is known that all of a man's affairs — both his acts and his thoughts, all will ascend above, before the L-rd, as we say in the Rosh Hashanah prayer: "For the remembrance of every creation comes before You, the acts of a man, etc., the thoughts of a man and his stratagems." And all will be set forth before the man, as it is written (Psalms 54:21): "I shall reprove you and I shall set it before your eyes," and the man will be greatly shamed.