Shemirat HaLashon שמירת הלשון

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.

To what can this be compared? To [the instance of] a wealthy man, a merchant in precious gems, who, journeying from home, asks one of his companions to take care of his possessions, permitting him first to see his beautiful gems. Upon opening the chest, he sees them, and, beside them rotted earth. At this, he says to himself: "This wealthy one is a fool! How can he leave rotted earth together with such beautiful jewels!" The analogue is self-explanatory. Is it not obvious that all the dead things of the world, in the course of years, all turn to dust, both he himself and all the things he yearned for in his lifetime, as it is written (Koheleth 3:20): "All came from dust and all returns to dust." And he will be astonished at himself forever. How could he have combined in his chest [i.e., in his heart] two loves together? Love of the L-rd and His Torah, the true gems, more desirable than gold and fine gold, and the love of dust — together! Did he not know in his lifetime that in the end all would return to dust? Therefore, one must take great care to drive out of his mind, thoughts of lust for the desires of the world. This is the intent of "And make our hearts one … and let us not be ashamed, forever." And this is the intent, in the recital of Shema, of (Devarim 6:5): "And you shall love the L-rd your G-d with all your heart, etc."