A symbol is indeed the only possible expression of some invisible essence, a transparent lamp about a spiritual flame; while allegory is one of many possible representations of an embodied thing, or familiar principle, and belongs to fancy and not to imagination: the one is a revelation, the other an amusement.
I had this thought a while ago,
"My darling cannot understand
What I have done, or what would do
In this blind bitter land."
And I grew weary of the sun
I can exchange opinion with any neighbouring mind,
I have as healthy flesh and blood as any rhymer's had,
But O! my Heart could bear no more when the upland caught the wind;
I ran, I ran, from my love's side because my Heart went mad.
Shakespeare cared little for the State, the source of all our judgments, apart from its shows and splendours, its turmoils and battles, its flamings out of the uncivilized heart.