THE THIRD BOOKE OF THE FAERIE QUEENE
It sited was in fruitfull soyle of old,
And girt in with two walles on either side;
The one of yron, the other of bright gold,
That none might thorough breake, nor over-stride:
And double gates it had, which opened wide,
By which both in and out men moten pas;
Th'one faire and fresh, the other old and dride:
Old Genius the porter of them was,
Old Genius, the which a double nature has.
He letteth in, he letteth out to wend,
All that to come into the world desire;
A thousand thousand naked babes attend
About him day and night, which doe require,
That he with fleshly weedes would them attire:
Such as him list, such as eternall fate
Ordained hath, he clothes with sinfull mire,
And sendeth forth to live in mortall state,
Till they againe returne backe by the hinder gate."
Blake used this (timeless idea) in one of his earliest works, Thel; in Plate 6:
"The eternal gates terrific porter lifted up the northern bar". (Erdman, page 6)
He used the Northern (down to earth) and Southern Gates more pointedly in The Arlington Tempera. Look closely and you may see Thel at the bottom of the northern stairs with her pail still full; she's seen more than she wants to and she's purposefully going back against the stream of the nymphs heading for mortality.
The Angel is pointing the traveler back up the Southern Gate; he has tasted mortality fully and is ready to go home.
Nevertheless with the Enlightenment this sort of idea had fallen into obscurity in most of the materialistic and rational minds of England. Bacon, Newton, and Locke were the primary exponents of rationalism in Blake's day. This meant in reality that no one was interested in the kind of poetry and philosophy that interested Blake.
From the Beyond (Eternity) the world was created; man was created; time and space were created; birth and death were created; good and evil are creatures, figments of a frail and created mind.. In the world: in man, time and space we perceive duality, or a multiplicity. In Eternity we imagine Unity.
The ultimate duality is between Eternity and the World, between God and man, but this is a sometime thing-- until the end of time. As a creature the world will end; you, too, will end, as a creature.
But the vision of the mystic suggests that you are more than a creature. The writer of Genesis had such an inkling when he described man as made of the dust of the earth, but in the image of God. The Quakers believe there is 'that of God' in everyone.
Eternal Death in Blake's language refers to the soul's descent from Eden (and Beulah) to the nether regions (Ulro) where Eternity is lost and only the created remains. Lost! but not forever; Eternal Death dies, too; Eternity waits for the soul's Awakening, which may be at the moment of mortal death.