John Calvin Commentary Isaiah 21:6

"For thus hath the Lord said unto me, Go, set a watchman: let him declare what he seeth:" — Isaiah 21:6 (ASV)

For thus has the Lord said to me. The Prophet is commanded to set a watchman on the watchtower, to see these things at a distance, for they cannot be perceived by the eyes or learned by conjecture. Therefore, so that all may know that he did not speak at random, he declares that he foretells these things. For although they are unknown to men and incredible, he clearly and distinctly knows them by the spirit of prophecy, because he is elevated above human judgment.

This should be carefully observed, for we must not imagine that the prophets learned from men or foresaw by their own insight what they made known; and on this account also they were justly called “Seers” (1 Samuel 9:9). Though we also see them, yet our sight is dull, and we scarcely perceive what is at our feet; even the most acute men are often in darkness, because they understand nothing but what they can gather by the use of reason.

But the prophets speak by the Spirit of God, as from heaven. The sum of what is stated is that whoever attempts to measure this prophecy by their own judgment will do wrong, because it has proceeded from God and therefore goes far beyond our perception.

Go, appoint a watchman. It gives additional weight that he “appoints a watchman in the name of God.” If it is objected, “You relate incredible things as if they had actually happened,” he replies that he does not declare them at random. For the one whom the prince has appointed to be a watchman sees from a distance what others do not know. So, Isaiah saw by the revelation of the Spirit what was unknown to others.