Wikipedia Paradise Lost Illustration 12, 1807
The forbidden tree in the Garden is not the tree of good and evil but is the Tree of Knowledge of Good and Evil. Without the knowledge of divisions there is no duality. This knowledge once acquired has a cascading effect which colors every perception. The mind falls into a trap of associating one aspect of each pair of alternatives with the good; the other with not good.
Consciousness of duality has been entered into and it seems the only substance.
 So he drove out the man; and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life.
These are the four into which the unity of Eden has been divided:
Serpent - Luvah
Dragon - Urizen
Vegetated Tongue - Tharmas
It is the western gate leading into Eden which is closed, the gate of Tharmas who has become the Vegetated Tongue. The integrating force has become the disintegrating force. It is a vegetating, devouring, false, revolving watery flame.
As the gate into Eden is closed the gates to the Vegetative World are opened. The sons and daughters of Los carry within themselves a 'Universe' of 'Translucent Wonder'. In the loins, heart, and head are gates through which they may pass to the breadth, to the height and depth, and inward; outward they cannot go. (They cannot return to Eden and enjoy immortality in the vegetative World.)
Jerusalem, Plate 13, (E 158) "But to those who enter into them they seem the only substances For every thing exists & not one sigh nor smile nor tear, Plate 14 One hair nor particle of dust, not one can pass away. He views the Cherub at the Tree of Life, also the Serpent, Orc the first born coild in the south: the Dragon Urizen: Tharmas the Vegetated Tongue even the Devouring Tongue: A threefold region, a false brain: a false heart: And false bowels: altogether composing the False Tongue, Beneath Beulah: as a watry flame revolving every way And as dark roots and stems: a Forest of affliction, growing In seas of sorrow. ... And Los beheld his Sons, and he beheld his Daughters: Every one a translucent Wonder: a Universe within, Increasing inwards, into length and breadth, and heighth: Starry & glorious: and they every one in their bright loins: Have a beautiful golden gate which opens into the vegetative world: And every one a gate of rubies & all sorts of precious stones In their translucent hearts, which opens into the vegetative world: And every one a gate of iron dreadful and wonderful, In their translucent heads, which opens into the vegetative world And every one has the three regions Childhood: Manhood: & Age: But the gate of the tongue: the western gate in them is clos'd, Having a wall builded against it: and thereby the gates Eastward & Southward & Northward, are incircled with flaming fires. And the North is Breadth, the South is Heighth & Depth: The East is Inwards: & the West is Outwards every way."
Another post on the Western Gate.