Moussar sur Les Nombres 15:39

The fourth principle is pain in [his] actions, as it is stated (Joel 2:12), "'Yet even now' - says the Lord - 'Turn back to Me with all your hearts, and with fasting, weeping, and lamenting." And they, may their memory be blessed, said (Yerushalmi Berakhot 1:4), "The heart and the eyes are the two intermediaries of sin." And so is it written (Numbers 15:39), "and you shall not go astray after your hearts and after your eyes." Hence with this will the sin of the intermediaries be atoned - with the measure of repentance according to the measure of the affliction. For the iniquity of the heart is atoned by its bitterness and sighing - with the brokenness with which it is broken, as it is stated (Isaiah 57:16), "when spirits in front of Me cover themselves." And it is [also] written (Psalms 51:19), "God, You will not despise a contrite and crushed heart." And the parable about this is from the [impure] vessels - when they are broken, they are purified, as it is stated (Leviticus 11:35), "an oven or stove shall be smashed." And the iniquity of the eyes is atoned by tears, as it is stated (Psalms 119:136), "My eyes shed streams of water, because they do not obey Your Torah ." It does not state, "because I do not obey Your Torah," but rather states, "they do not obey"; since they were what caused the sin - therefore, I shed streams of waters.

And this is the intent of (Bamidbar 15:40): So that you remember and do all of My mitzvoth and you will be holy to your G-d." Now, ostensibly, "so that you remember and do all of My mitzvoth" is superfluous, for it is written [just] before (Ibid. 39): "and you will remember all the mitzvoth of the L-rd and you will do them." And it should have been written there "and you will be holy to your G-d." But, the verse [39] intimates to us something essential, i.e., when are the mitzvoth of so much avail that through them a man will be "holy to G-d"? When he takes heed not to go astray after the thoughts of his heart and the sight of his eyes. As Chazal have said on this verse, (Berachoth 12): "'After your hearts' — this is heresy;" 'and after your eyes' — this is adultery.'" And the Chinuch writes that included in "heresy" are all of the thoughts that run counter to the perspective of Torah, and included in "adultery" is pursuing the lusts of the world.

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.