Naturally expositors have various conjectures and the opinions are many. The main theories seem to be these:
- A quotation from Mica in the margin of Isaiah accidentally transferred into the main text
- Both quote another prophecy
- Both independently receive the same inspired prophecy
- Either one quotes the other
This debate goes way back and Joseph Alexander tallies up some big names who debated the issue:
These verses are found, with very little variation, in the fourth chapter of Micah (vers. 1–3), to explain which some suppose, that a motto or quotation has been accidentally transferred from the margin to the text of Isaiah (Justi, Eichhorn, Bertholdt, Credner); others, that both Prophets quote from Joel (Vogel, Hitzig, Ewald); others, that both quote from an older writer now unknown (Koppe, Rosenmüller, Maurer, De Wette, Knobel); others that Micah quotes from Isaiah (Vitringa, Lowth, Beckhaus, Umbreit); others, that Isaiah quotes from Micah (J. D. Michaelis, Gesenius, Hendewerk, Henderson). This diversity of judgment may at least suffice to shew how vain conjecture is in such a case. The close connection of the passage with the context, as it stands in Micah, somewhat favours the conclusion that Isaiah took the text or theme of his prediction from the younger though contemporary prophet. The verbal variations may be best explained, however, by supposing that they both adopted a traditional prediction current among the people in their day, or that both received the words directly from the Holy Spirit. So long as we have reason to regard both places as authentic and inspired, it matters little what is the literary history of either. (Alexander, J. A. (Trans.). (1870). The Prophecies of Isaiah Translated and Explained, Vol. 1, p. 96)