'It is we alone who have devised cause, sequence, reciprocity, relativity, constraint, number, law, freedom, motive and purpose; and when we interpret and intermix this symbol-world, as 'being in itself, with things, we act once more as we have always acted-mythologically.' F. Nietzsche, Beyond Good and Evil
Symbolism is the art of defining higher ideas of this existence through visual, comprehensible effect, or tangible form that is manifest in religious, literary, and mythological content. This is precisely Blake's intent when he visualizes his 'Eternal Reality', and precisely what occurs when he brings the 'Divine Humanity' into personification through the 'Fourfold Man'. Although many consider Blake a Romantic anomaly, who acts without prior foundation, there are others (like Kathleen Raine, for example) who have pioneered a perspective which suggests that Blake actually operated on a solid tradition, drawing from platonic sources such as Plotinus, Porphyry, and Thomas Taylor the Platonist, as well as from alchemical, hermetic, and mystical sources such as the Corpus Hermeticum, Paracelsus, and Jacob Boehme. The main similarity between these two strains of ideas is their common utility of symbolism; they share the same alchemical images, constituting the primary elements which Blake borrows on a constant basis, some of which can be traced as early as his 'Songs of Innocence and Experience'.
Now, regarding the symbolism itself, Blake identifies both explicit and implicit themes of the human psychological strata and summarily attaches a distinctly human personification to that particular theme, which exemplifies not only its apparent characteristics, but essential function and, most importantly for Blake, its distorted function after the Fall. Blake assumes a mostly orthodox perception of the metaphorical 'fall' of mankind: Albion, the original creative energy, imagines a beautiful, feminine image that he himself becomes delighted and entranced with, and in so doing supposes that she has a will outside of himself; and thus the generation of the 'Female Will', and thus the genesis of 'Ulro', the world of generation.
However much this essentially complies with the traditional conception of the Fall, Blake does make a few of his own modifications; the main one being a claim that the fall of divinity and the creation of the physical world were the same event; that they were indeed simultaneous. Knowing that all reality is mental, the perception of a physical dimension already alerts us to the evidence that our perception is fallen, incomplete, and derivative; the natural consequences of this crisis obviously lead us to believe that the decline in our perceptive faculty is continuous, a theory that is easily ratified by the demise of mythological hierophany, the ever more limited understanding of the 'symbolic world', 'personalized' theology, scientific abstraction, et cetera. Despite the diminution of divinity perceptible to ordinary understanding, imagination is clearly extant and, especially for Blake, manifests itself in universal symbolic format, allowing us a visual foundation upon which we can rebuild true, primordial perception.
'Whereas the philosophes saw Christianity as merely symbolic, Blake saw it as supremely symbolic. Dogmatic doubt hid from their eyes the philosophic myth.' - Leopold Damrosch Jr.
The above quote implies another area of disagreement between Blake himself and not only with the philosophes, the neoclassical Augustans, the reasoners and doubters of his time, but a disagreement also with contemporary theologians, dogmatists, and moralists. Amongst countless other items, Blake opposed the literal historicism of latter-day scholars, who naively reconstruct a Biblical dogma using only the hard data of the scripture, failing to perceive the symbolic and imaginative significance of its content, which inevitably results in an oppressive, inflexible morality, and a lifeless, absolutist, ugly theology. Blake's view inherits Dante's idea of anagogy, where the scripture is interpreted as it relates to 'prophetic vision', which necessarily incorporates the higher principles of 'Eternal Reality'. The Blakean symbols everywhere visualize ideas and themes of these 'higher principles', whether they refer to certain elements of epistemology, alchemy, psychology, eschatology, and so on. And, finally, as Damrosch says, 'Symbols are mental constructs which create, rather than mirror, the reality they claim to represent'. So symbolic reality attains a greater ontological status than the very state which they represent due to their a priori configuration of mental perception; we define reality, and we do this best through visionary, mental perception, not sensual feeling vis-a-vis Lockean empiricism.