I’m accustomed to the ESV Isaiah 1:18 (defining my own a/b/c clauses for the sake of discussion):
(a): “Come now, let us reason together, says the LORD:
(b): though your sins are like scarlet, they shall be as white as snow;
(c): though they are red like crimson, they shall become like wool.
"Let us reason together" (a) has always confused me because what follows (b, c) seems particularly unreasonable.
I noticed a footnote in the ESV on "reason": or dispute. That’s...different. The Hebrew (BHS):
לְכוּ־נָ֛א וְנִוָּֽכְחָ֖ה יֹאמַ֣ר יְהוָ֑ה אִם־יִֽהְי֨וּ חֲטָאֵיכֶ֤ם כַּשָּׁנִים֙ כַּשֶּׁ֣לֶג יַלְבִּ֔ינוּ אִם־יַאְדִּ֥ימוּ כַתֹּולָ֖ע כַּצֶּ֥מֶר יִהְיֽוּ׃
The HALOT entry for the word in question, from root יכח:1

“Arguing (in a lawsuit)” has a different sense than “reasoning (together).” Neither concept strikes me as very likely to result in though your sins are like scarlet, they shall be white as snow. The Anchor Bible translation (Joseph Blenkinsopp) also uses “reason together” but adds an interesting twist by understanding (b, c) as questions:
(b) If your sins are colored scarlet, can they become white like snow?
(c) If dyed crimson red, can they become like pure wool?
I haven’t run across a standard translation that makes that decision, but it’s appealing to me as a (sarcastic?) elaboration of the “reasoning” introduced in (a) and consistent with the conditionals that come next in v 19-20. (If you are willing...but if you refuse... (vv. 19-20) after [Your sins] shall be white as snow seems like a non sequitur to me.)
- What is the sense of יכח in this context?
- Are these possibly questions (a la Blenkinsopp)?
- Is there a better way to understand how (b) and (c) follow from (a)?
1. Included here is the nifil excerpt only; note that the cross indicates that these three instances comprise a complete listing of the usages (of this stem) in the Hebrew bible.