Source: https://bitesizedexegesis.com/2016/09/16/bite-sized-exegesis-proverbs-1021/
Timestamp: 2019-04-24 00:07:56+00:00

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The lips of a righteous man will shepherd many, but foolish people by lack of mind will die.
Like Proverbs 10:20, this verse works just fine by itself, but the two verses appear to work even better as a couplet. Meter is hard to measure, but it appears to me to be the same in both verses.
Certainly the number of words in each clause is the same in both verses (4+3), if you count baḥăsar-lēb as a single word, and I think one should. This is both because of the maqqef and because of the use of ḥăsar-lēb as, it appears, a compound word throughout the Hebrew Bible.
There is a aural similarity between the last word of each verse: kimǝʿāṭ and yāmûtû both prominently feature ‘m’ and ‘t’ sounds. There is also a vaguer aural similarity between the first words of each verse: kesep and śipǝtê both feature ‘s’ and spirantized ‘p’ sounds.
The subject matter certainly is similar. Both contrast the value of the public expression of a wise man’s thoughts (‘tongue’ in v. 20, ‘lips’ in v. 21) with the lack of public value of the unexpressed thoughts (lēb = heart/mind) of a contrasting group (‘wicked people’ in v. 20, ‘fools’ in v.21). But this, in itself, doesn’t argue for the two verses being a couplet. But in addition to their basic similarity, the second saying seems to develop the first one somewhat.
Verse 20 says that what a righteous man says is invaluable, while the inner thoughts of wicked people are worthless. Now verse 21 moves on to more a more specific description of the value of a righteous man’s words: they shepherd many. The image is of feeding and tending, causing to pasture. The righteous man’s words are priceless because they feed the souls of multitudes. Righteous men speak good things into many lives: encouragement, wisdom, and meaning. A righteous man will speak the wisdom of YHWH, and this brings life to those who will receive it. This thought is certainly embedded in v. 20, but v. 21 fleshes it out in a complementary fashion.
The second clause is also developed. While the thoughts (lēb) of wicked people are without value, fools actually lack thoughtfulness (ḥăsar-lēb) altogether, and they perish because of it. These two verses taken together actually seem, like v. 18, to be saying that wicked people are bad enough, but fools are even worse.
First, the theory is that there is a marked tendency to avoid diagonal contrasts, not a rule. So given that this is the first saying out of 22 so far to clearly do so doesn’t discount that observation.
Second, my survey of the rest of the book has so far not produced another clear instance of diagonal contrast. There are other concerns in Proverbs: diligence versus laziness and wealth versus poverty, to name the most important. But even with these extra categories I am not seeing a tendency towards diagonal comparisons/contrasts. A closer examination of the rest of the book will determine if the theory holds water.
Third, the connection between righteousness and wisdom is very strong in the first half of this verse (and in the preceding verse). Seeing that a progression is intended in the couplet of vv. 20 and 21 (wicked people are bad enough, but fools are even worse), the contrast really is between the righteousness/wisdom combination and the wickedness/foolishness combination, rather than between righteousness and foolishness, strictly speaking.
A translation and commentary of Proverbs 10:21 - "The lips of a righteous person will shepherd many, but fools will die for want of thoughtfulness."

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