'The vegetable universe, opens like a flower from the Earth's center:
In which is Eternity. It expands in stars to the Mundane ShellAnd there it meets Eternity again, both within and without,And the abstract voids between the stars are the Satanic Wheels.' Jerusalem
The most profoundly Platonic concept in Blake is, of course, the metaphysical aspect, which Plato grasps through the cultivation of higher reason, and Blake through imaginative vision. Plato and his followers down through history intuit a preconditioned, causal realm that is the primal source of all real existence, and its Ideas are the underlying, perfect forms of every apparent object in this world. Blake sees the same thing, visualizing eternal forms that persist in time in diverse identities but essentially unchanging.
'The Nature of Visionary Fancy or Imagination is very little known & the eternal nature & permanence of its ever Existent Images is considered as less permanent than the things Vegetable and Generative Nature yet the Oak dies as well as the Lettuce but its Eternal Image &Individuality never dies, but renews by its seed. Just so the Imaginative Image returns by the seed of Contemplative Thought.' Vision of the Last Judgment
Although Blake violently refused the tendency to abstract and generalize 'Minute Particulars' into general forms, he does not reject universals outright, merely the categorization of many varying individuals into one bloated aggregate that he found to be capriciously constructed and intellectually revolting. Universals exist, that is apparent, but, in reference to platonic thought, 'General forms have their vitality in Particulars'. So, while Plato assumed the universal forms to be multiple, Blake supposes only one universal form, namely, the 'Divine Humanity', from whence all others derive. The divine humanity is constituted by members of every type, all very much aware of their identity in the greater whole; simply stated, Plato put prominence on the forms, or the entirety of the universal, while Blake emphasized the distinction and particular merit of individuals and how they relate to the universal..
Another important platonic theme that Blake inherits is found in the common imagery that they share. For instance, Plato and his intellectual posterity discern certain alchemical motifs in traditional myth and literature; one being Plotinus' observation of Odysseus (nominally figured as a kind of 'lesser hero' for his low cunning and trickery) as the very 'type and image', marking him as the quintessential fallen man. He makes this distinction by virtue of the hero's displacement from Ithaca, his home, and his longing to return, and, when he is finally returned, he sheds his disguising rags just as the human sheds his body upon entrance into heaven. Moreover, the caves and water in general are widely depicted as 'female powers', associated with the purely physical, chaotic portion of of our world. Porphyry asks the question, 'What's water but the generated soul?...for souls descending into generation fly to moisture'. And Heraclitus, who prefigures a lot of platonic ideas: 'a dry soul is the wisest... moisture appears delightful to souls', although it is fatal to them. The seas are sun-less and storm-tossed, being bereft of spiritual light. When an image falls asleep, it has lost its vision of eternity, which initializes the sufferings of a Job, or an Odysseus. Raine says that, 'the symbolic event is parallel; for the separation of Satan, the human Selfhood, from the divine world is about to actuate such a cycle of Experience as the Platonic lapse of the soul'.
These primordial ideas captured by the Platonic tradition are rejuvenated in Blake, who enriches his symbolic world with lively allusions to these arcane images; the most obvious parallel is the giant figure Albion, who is constantly depicted as asleep, groaning, desperately desirous of some place he has little conscious idea of. The long poem 'The Book of Thel', which asks why souls must be birthed and placed into mortal bodies, is largely based off of the Platonic descent of the soul. The lovely Thel, upon viewing the terrors of the generated world, takes Plotinus' advice (figuratively speaking, of course) and retreats back 'into the vales of Har'. The poem 'Luvah', the converse of 'The Book of Thel', tells of Vala who, lured by the seeming wonders of earth, is led by the erotic Luvah, who shows her his utopia of shadows; but when Vala loses Luvah (a highly symbolic and necessary event), she has nothing left but a vacant world of endless toil and perpetual dusk. Blake is ultimately dissatisfied with both Thel and Vala, and it is not until his later works that his own vision is truly complete; when he has finally discovered his own particular identity in the great myriad of multiplicity.