ציטירנדיקער פּירוש אויף ירמיהו 31:13

The Mechilta also mentions that the word אז is sometimes used in the future tense and sometimes in the past tense. Examples of the word occurring in the past tense are Genesis 4,26, Exodus 4,26, Numbers 21,17, Joshua 10,12 and others. At the same time there are numerous examples of the word אז referring to future events such as in Isaiah 60,5 אז תראי ונהרת, Isaiah 58,8 אז יבקע כשחר אורך, Isaiah 35,6 אז ידלג כאיל and many others. Moses reasoned that since he had used the word אזwhen he had criticised G’d’s apparent indifference to Pharaoh’s additional decrees worsening the condition of the Jewish slaves (Exodus 4,10), it was now up to him to make amends by using the same word in exalting Him. The word אז, i.e. 1+7, is an allusion to the Unique One who reigns supreme above the seven heavens.
According to a Midrash Tanchuma on Genesis 4,26 the Israelites reasoned that seeing that word had been used when G’d punished mankind for the first time collectively when He caused one third of the dry land mass to be inundated in response to the idolatry introduced by the generation of Enosh, it was appropriate to use the same word at a time when G’d’s sovereignty on earth had been demonstrated by means of the water which had been made to retreat even from its own habitat.
The words משה ובני ישראל לאמור, teach that Moses and the Children of Israel had the good fortune to personally use this formula of exalting G’d, i.e. praising the attribute called זאת לה'. This was like the bride who applauds her husband the King and says: שיר השירים אשר לשלמה. [This mystical approach is based on the apparent redundancy otherwise of the extra words את השירה הזאת. Ed.] The author promises to deal with this in greater detail in Numbers 21,19.

L’MA’AN’ (THAT) THE WATERED BE SWEPT AWAY WITH THE THIRSTY. The meaning thereof is “‘to supplement’ the watered with the thirsty,” for the satisfied soul is called ravah (satiated), similar to, And I will ‘satiate the soul’ of the priests with fatness, and My people shall be satisfied with My goodness;³⁰Jeremiah 31:13. and their soul shall be as a watered garden,³¹Ibid., Verse 11. while the desiring soul is called tz’meiah (thirsty): My ‘soul thirsteth’ for Thee.³²Psalms 63:2. Therefore, the sense of the verse is: when into a person’s satisfied soul — one that does not desire harmful things — there comes a slight craving, and he satisfies his lust, then there will grow in his soul a craving even greater than the original one, for the thing that he ate or that he did, and it [the soul] will lust [even] after evil things for which he never craved at first. Thus, if someone craves for lewdness with beautiful women and becomes carried away in unchastity, his lust will ultimately bring him to sodomy and to bestiality and the like with other forms of lust. This is akin to what the Sages have mentioned [with respect to sexual craving]:³³Succah 52b. “The more one satisfies it, the more it hungers, the more one starves it, the more it is satisfied.” Therefore Scripture states concerning him who follows the stubborness of his heart, that if he will satisfy his soul with those stubborn and overpowering desires for which it thirsts, his soul “will add the watered to the thirsty,” for then he will desire and thirst after things for which he had been satisfied, and with which his soul was satiated [meaning that, up to then, he felt no desire for them]. And, because he used the expression ‘shoresh poreh’ (a root that beareth),³⁴Verse 17. he follows this usage, saying that this root shall add a branch that will be satiated and leafy to the thirsty one and they will all become thirsty. Thus the word l’ma’an (that) in this place does not refer to the reason [i.e., “so that”]: “I will go in the stubborness of my heart so that I supplement the watered with the thirsty.” Instead its meaning is like “to add” — Scripture is saying that because of his conduct, this will be the result.³⁵In other words, it is not the sinner who says: “L’ma’an (so that) I supplement the watered etc.” but it is Scripture which declares “L’ma’an etc.” that “to add” the watered etc., will result from his conduct. For in every result of a particular action, Scripture uses the term l’ma’an, whether the responsible person does it intentionally to effect that result, as ‘l’ma’an’ (so that) it may be well with us,³⁶Jeremiah 42:6. See my Hebrew commentary p. 478, Note 32, that the reference in the Hebrew text of Ramban is to above, 4:40, but the word l’ma’an is not found there in connection with this expression. or whether the person effecting the result does so unintentionally, as in the expressions: ‘l’ma’an’ (that) their heart may melt;³⁷Ezekiel 21:20. ‘l’ma’an’ (that) your altars may be laid waste;³⁸Ibid., 6:6. Of their silver and their gold have they made them idols, ‘l’ma’an’ (that) they may be cut off,³⁹Hosea 8:4. The result referred to in each of the last three verses is involuntary. Here, too, it cannot be said that they made idols of gold and silver “so that” they may be cut off. Rather, Scripture states that the undesired result will be that, in punishment, they will be cut off. and many other similar verses. In my opinion this is the best and most correct of all that has been written about this verse from many aspects.
It is also possible to explain that the phrase [that the watered …] refers back to the expression a root that beareth gall and wormwood,³³Succah 52b. the verse stating that he will bless himself in his heart, saying, ‘I shall have peace,’ so that his watered branch, on which grow and sprout the wormwood and gall, should be added to the thirsty one on which nothing has grown.⁴⁰The difference between the two interpretations is: In the first interpretation “the watered” was not lustful — it was “the thirsty” that was the sinful part of the soul. In the second interpretation, however, the sinful aspect of the soul is “the watered” part, for on it has already grown “the gall and the wormwood;” and “the thirsty” is the pure part of the soul, for nothing evil has yet grown on it. The third explanation which follows is similar to the second with a minor change, as explained in the text (Kur Zahav). Or the watered may be a surname for the aforementioned wormwood, meaning that the watered wormwood which sprouts and grows exceedingly will be added to the thirsty which until now has grown but little — so that now both will sprout. Thus the intent is to state that anyone who will hear the imprecation and will imagine himself to be immune against it, holding strongly to [this] stubborness, and, as a result, his thoughts will bring him to [further] sin sprouting transgressions greater than the first [the Eternal will not be willing to pardon him etc.].²⁶Verse 19. This also is a correct interpretation.