imagination bodies forth
The forms of things unknown, the poet's pen
Turns them to shapes, and gives to airy nothing
A local habitation and a name.

And a little before he had told us that

Lovers and madmen have such seething brains,
Such shaping fantasies, that apprehend
More than cool reason ever comprehends.

Plato had said before him (in his "Ion") that the poet is possessed by a
spirit not his own, and that he cannot poetize while he has a particle
of understanding left. Again he says that the bacchantes, possessed by
the god, drink milk and honey from the rivers, and cannot believe, _till
they recover their senses_, that they have been drinking mere water.
Empedocles said that "the mind could only conceive of fire by being
fire."

All these definitions imply in the imaginative faculty the capabilities
of ecstasy and possession, that is, of projecting itself into the very
consciousness of its object, and again of being so wholly possessed by
the emotion of its object that in expression it takes unconsciously the
tone, the color, and the temperature thereof. Shakespeare is the highest
example of this--for example, the parting of Romeo and Juliet. There the
poet is so possessed by the situation, has so mingled his own
consciousness with that of the lovers, that all nature is infected too,
and is full of partings:

Look, love, what envious streaks
Do lace the _severing_ clouds in yonder east.

In Shelley's "Cenci," on the other hand, we have an instance of the
poet's imagination giving away its own consciousness to the object
contemplated, in this case an inanimate one.

Two miles on this side of the fort, the road
Crosses a deep ravine; 't is rough and narrow,
And winds with short turns down the precipice;
And in its depth there is a mighty rock
Which has, from unimaginable years,
Sustained itself with terror and with toil
Over a gulf, and with the agony
With which it clings seems slowly coming down;
Even as a wretched soul hour after hour
Clings to the mass of life; yet clinging, leans;
And leaning, makes more dark the dread abyss
In which it fears to fall: beneath this crag,
Huge as despair, as if in weariness,
The melancholy mountain yawns.

The hint of this Shelley took from a passage in the second act of
Calderon's "Purgatorio de San Patricio."

No ves ese penasco que parece
Que se esta sustentando con trabajo,
Y con el ansia misma que padece