Hughes tells us that his tutor of Blake was Greville MacDonald, writer of The Sanity of William Blake and son of George MacDonald.
Hughes suggested that "In moments of illumination Blake attained fullness of vision and saw four worlds but he knew that only one was eternal; the other three were built of mingled shadow and faith; their eyes were not yet opened and healed by the touch of the divine mercy."
Then he found this key in Jerusalem:
"to open the immortal Eyes
Of man inwards, into the worlds of Thought, into Eternity
Ever expanding into the Bosom of God--the Human Imagination"
From this Hughes (and I) conclude that Blake meant that we all have 'immortal Eyes', like baby chicks with eyes not yet open.
Hughes continues: "Blake had been entrusted with a mighty visual imagination and with a poetic ability in the use of words, and both must be dedicated unreservedly to this great task. And so he labored year after to write Jerusalem for us."
After that work he turned away from words and began painting pictures.