C. F. Keil and Franz Delitzsch on Isaiah 21:6

Isa 21:6

The prophecy is continued with the conjunction “for” (ci). The tacit link in the train of thought is this: they act thus in Babylon, because the destruction of Babylon is determined. The form in which this thought is embodied is the following: the prophet receives instruction in the vision to set a metzappeh upon the watch-tower, who was to look out and see what more took place. “For thus said the Lord to me, Go, set a spy; what he seeth, let him declare.” In other cases it is the prophet himself who stands upon the watch-tower (Isa 21:11; Hab 2:1-2); but here in the vision a distinction is made between the prophet and the person whom he stations upon the watch-tower (specula). The prophet divides himself, as it were, into two persons (compare Isa 18:4 for the introduction; and for the expression “go,” Isa 20:2). He now sees through the medium of a spy, just as Zechariah sees by means of the angel speaking in him; with this difference, however, that here the spy is the instrument employed by the prophet, whereas there the prophet is the instrument employed by the angel.