Albert Barnes Commentary Isaiah 6:9

"And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not." — Isaiah 6:9 (ASV)

And he said ... - The expressions which follow are those which denote hardness of heart and blindness of mind. They would hear the words of the prophet, but they would not understand him. They were so obstinately bent on iniquity that they would neither believe nor regard him. This shows the spirit with which ministers must deliver the message of God. It is their business to deliver the message, though they should know that it will neither be understood nor believed.

Hear you indeed - Hebrew ‘In hearing, hear.’ This is a mode of expressing emphasis. This passage is quoted in Matthew 13:14; see the note at that place.