Any attempt to explain Blake’s symbolism will involve explaining his conception of symbolism. To make this clear we need Blake’s own definition of poetry:
Allegory addressed to the Intellectual powers, while it is altogether hidden from the Corporeal Understanding, is My Definition of the Most Sublime Poetry; it is also somewhat in the same manner defined by Plato.
The “corporeal understanding”, according to Blake, cannot do more than elucidate the genuine obscurities, the things requiring special knowledge to understand (such as the contemporary allusions in Dante), or the literal mechanics of a poem (meter, structure, general themes etc). The “intellectual powers” go to work rather differently: they start with the hypothesis that the poem in front of them is an imaginative whole, a unique and irreplaceable event, and work out the implications of that hypothesis. The way that poetry is generally taught in schools therefore, by converting it into “corporeal understanding” – into a form of machinery – completely misses its whole point, like explaining a joke or analysing a dead body to find out what makes it tick.