The phrase “starry pole” is a quotation from Milton’s Paradise Lost, whose book IV describes the life of Adam and Eve in the garden of Eden:
Thus, at their shady lodge arrived, both stood,
Both turned, and under open sky adored
The God that made both sky, air, earth, and heaven,
Which they beheld, the moon’s resplendent globe,
And starry pole.
John Milton (1674). Paradise Lost, IV.720–725. Spelling modernized.
By “starry pole” Milton means the celestial pole, the part of the sky about which the stars appear to revolve, containing the pole star, which appears fixed in the sky. In Milton’s cosmology, the “empyrean” or “heaven of heavens” lies above the pole, and he sometimes uses “pole” as a metonym for heaven (for example in VII.215, “with the centre mix the pole”).
With this quotation in mind, we can see that Blake is alluding to the biblical story of the fall of man in Genesis chapters 2 and 3. Blake’s “ancient trees” are the trees in the Garden of Eden. The “Holy Word” is Jesus (John 1:1: “In the beginning was the Word”) who walked in Eden and spoke to Adam and Eve (Paradise Lost X.97–98: “The voice of God they heard / Now walking in the garden”). “Lapsed” (meaning “fallen, sinful”) refers to the state of mankind after the fall. Blake’s bard tells the lapsed soul to “control the starry pole”: that is, to change the direction of rotation of the heavens, and turn back time, until the fall is reversed, and “light”, that is, innocence, is renewed.
“Fallen light” might also refer to the hero of Paradise Lost, that is, to Lucifer, whose name means “light-bringer”, and who “fell from noon to dewy eve” in I.743. (Compare Milton’s “dewy eve” to Blake’s “evening dew”.) If time were turned back, that might reverse not only the fall of man, but the fall of the angels too.
Why Geoffrey Keynes thought that the “starry pole” represented “Reason” I do not know: in context he provides no further details or explanation. It is true that Blake was interested in the contrast between reason and imagination (see extract from one of his letters below), but I do not see how this led Keynes to his interpretation of ‘Introduction’.
I know that This World Is a World of imagination & Vision. I see Every thing I paint In This World, but Every body does not see alike. To the Eyes of a Miser a Guinea is more beautiful than the Sun, & a bag worn with the use of Money has more beautiful proportions than a Vine filled with Grapes. The tree which moves some to tears of joy is in the Eyes of others only a Green thing that stands in the way. Some See Nature all Ridicule & Deformity, & by these I shall not regulate my proportions; & Some Scarce see Nature at all. But to the Eyes of the Man of Imagination, Nature is Imagination itself. As a man is, So he Sees. As the Eye is formed, such are its Powers. You certainly Mistake, when you say that the Visions of Fancy are not to be found in This World. To Me This World is all One continued Vision of Fancy or Imagination, & I feel Flatter’d when I am told so. What is it sets Homer, Virgil & Milton in so high a rank of Art? Why is the Bible more Entertaining & Instructive than any other book? Is it not because they are addressed to the Imagination, which is Spiritual Sensation, & but mediately to the Understanding or Reason? Such is True Painting, and such was alone valued by the Greeks & the best modern Artists. Consider what Lord Bacon says: “Sense sends over to Imagination before Reason have judged, & Reason sends over to Imagination before the Decree can be acted.”†
William Blake (23 August 1799). Letter to Dr. Trusler. In Geoffrey Keynes, ed. (1956). The Letters Of William Blake, pp. 35–36. London: Macmillan.
† Francis Bacon (1605), Of the Proficience and Advancement of Learning, Divine and Humane, book II, p. 47.