Blake's ability to handle multiple aspects of reality at multiple levels of existence leaves most of us mystified. In this little passage we have the Eternals who, though multiple, speak with one voice; we have four forms of Milton in different locations and performing different acts; we have Urizen being molded from the clay by Milton; we have Blake negotiating with God as did Moses when he got his assignment; we have the Seven Angels of the Presence and all they imply; and we have Orc in chains.
Blake's mind operates in this fragmented, multiple way but so does every mind and every complex system. To provide order - to make connections - is the task of finding the meaning in the chaos. Meaninglessness is an affliction of the mind which has not reached the level where there is a cohesive wholeness which can assemble the pieces into a picture. Collect the pieces, look at them in the light of your own experience, follow the threads to their origins, associate the pieces which fit together: gradually out of the indistinct haze clear 'lineaments' may come into focus.
Milton, Plate 20 ,(E 114)
"So spoke they as in one voice! Silent Milton stood before
The darkend Urizen; as the sculptor silent stands before
His forming image; he walks round it patient labouring.
Thus Milton stood forming bright Urizen, while his Mortal part
Sat frozen in the rock of Horeb: and his Redeemed portion,
Thus form'd the Clay of Urizen; but within that portion
His real Human walkd above in power and majesty
Tho darkend; and the Seven Angels of the Presence attended him.
O how can I with my gross tongue that cleaveth to the dust,
Tell of the Four-fold Man, in starry numbers fitly orderd
Or how can I with my cold hand of clay! But thou O Lord
Do with me as thou wilt! for I am nothing, and vanity.
If thou chuse to elect a worm, it shall remove the mountains.
For that portion namd the Elect: the Spectrous body of Milton:
Redounding from my left foot into Los's Mundane space,
Brooded over his Body in Horeb against the Resurrection
Preparing it for the Great Consummation; red the Cherub on Sinai
Glow'd; but in terrors folded round his clouds of blood.
At last when desperation almost tore his heart in twain
He recollected an old Prophecy in Eden recorded,
And often sung to the loud harp at the immortal feasts
That Milton of the Land of Albion should up ascend
Forwards from Ulro from the Vale of Felpham; and set free
Orc from his Chain of Jealousy, he started at the thought"
To add another wrinkle to this complex scenario Erdman brings our attention to Felpham's Vale and Blake's subsequent return to London. In commenting on Plate 20  of Milton, Erdman states in The Illuminated Blake (Page 238):
"The cheering thought is that Milton, whose descent to Felpham's vale is rehearsed from many angles in this poem, will then ascend 'up' and 'Forwards' (59-60). (The personal parallel is Blake's retreat from Lambeth to Felpham - a journey for which he borrows the sandals of Los in Lambeth [24: 9-11] - seen as preparation for his return to mental war in London. The two moves are the pulsations between which the whole poem lives.)"