In the beginning of the eighteenth century, imagination was not a cardinal point in poetical theory. For Pope and Johnson, as for Dryden before them, it has little importance, and when they mention it, it has a limited significance. What matters in poetry according to them is the truth to the emotions, or, as they prefer to say, sentiment. They prefer to speak in general terms and not to indulge themselves in creating new worlds.
For a whole century English philosophy had been dominated by the theories of John Locke who assumed that in perception the mind is wholly passive, a mere recorder of impressions from without, "a lazy looker on the external world." Both Locke and Newton found a place for God in their universes, the former on the ground that the works of nature in every part of them sufficiently evidence a deity, and the latter on the principle that the great machine of the world implies a mechanic.
Tom Paine, a well known thinker and a close friend to William Blake, assumed that the creations of the imagination are mere fantasies and, as such, divorced from life. In addition, William Shakespeare shows his acquaintance to this belief along with approval of an Italian philosopher called Picodella Mirandola, who thought that the imagination is almost a diseased faculty. Moreover, Francis Bacon in turn regarded imagination as a harmless and not an unpleasant activity, but not more. The position stated above is plainly unsatisfactory for poets who believe that the imagination is a divine faculty concerned with the central issues of the whole being, and it is extremely fundamental because without it there is no poetry.
English Romantics gave a great importance to imagination, so they were interested in images (visual impressions and metaphors). For the English Romantics, the belief in imagination was like the belief in individual self ; they admired sentiments. The mind is the central point and governing factor. The most vital activity of mind is imagination, and the source of spiritual energy is divine. They believed when they exercised imagination that they partake of the divine activity of God. Blake and Coleridge were the pioneers who insisted that the most vital activity of the mind is imagination, and they were hostile to the whole system of Locke and Newton.
For Blake the world of imagination is infinite and eternal whereas that world of generation, of vegetation, is finite and temporal. He also claimed that the imagination is nothing less than God as he operates in the human soul. It follows that any act of creation performed by the imagination is divine and that in the imagination man's spiritual nature is fully and finally realized.
As to Coleridge it is true that he regards poetry as a product of secondary imagination, but since this differs only in degree from the primary it remains clear that to him imagination is of first importance because it partakes the creativity (divine activity) of God. To him, imagination is related to truth and reality, and it is connected with a special insight. It sees things to which the ordinary intelligence is blind. Insight and imagination are inseparable, for they complete each other. Insight awake the imagination to work and is sharpened by it when it is at work.
Romantics combine imagination and truth because their creations are inspired and controlled by a peculiar insight. What matters to them was an insight into the nature of things. They refused Locke's limitation of perception to physical objects because it robbed the mind of its most essential function (perceive and create). It was this search for an unseen world which awakened the inspiration of the Romantics and made poets of them. However, imagination can't be considered an escape from life. Coleridge believed that imagination working with intuition can make discoveries on matters which really concern us.
The Romantics wanted to explore the world of spirit, so visible things aren't every thing unless they are related to an embracing power. They believed that through imagination and insight they could understand the things of spirit and present them in poetry. Apprehension of spiritual issues differs from scientific apprehension of natural laws or philosophical grasp of general truths. These laws and truths are stated in abstract words, but spiritual powers must be introduced through particular examples. When imagination is on them, we begin to understand their significance. In nature, Romantic poets found their initial inspiration. It wasn't everything to them, but they would have been nothing without it.
Coleridge had a deep trust in imagination as something which gives a shape to life. He believes that nature lives in us, and it is we who create all that matter in her. Although Coleridge is a little hampered by the presence of an external world, he feels in some way he must conform to it. Yet, when his creative genius is at work, it brushes these hesitations aside. Because he was fascinated by the notion of unearthly powers, he believed that the task of poetry is to convey the mystery of life, and it was their influence he sought to catch. Moreover, he believed that life is ruled by powers which can't be fully understood, so the result is a poetry more mysterious.
Wordsworth agreed with Coleridge on the distinction between imagination and fancy, for he believes that imagination is the most important gift a poet can have. Wordsworth didn't relate reason to anything, but he insisted that the inspired insight is itself rational. However, he differs from Coleridge in his conception of the external world; he accepts its independent existence and insists that imagination must in some sense conform to it. Moreover, he believes that imagination must somehow be related to the external world because that world is not dead but living and has its own soul and distinct from the soul of man, and man's task is to connect with this soul; man's life is shaped by nature . Wordsworth believed that he helped this soul of nature to become closer to man and could show how the external world and the individual mind fits each other. As concerning nature, it was the source of his inspiration. Wordsworth sought for a state in which the soul of nature should be united with the soul of man.
As for Shelley, he was also attached to imagination, but he saw that reason must somehow be related to imagination and believed (unlike Wordsworth) that its special task is to analyze the given and to act as an instrument for the imagination. Shelley calls poetry the "Expression of Imagination" because in it divers things are brought together instead of being separated through analysis. In his "Defense of Poetry" he claimed that the poet has a special kind of knowledge, a sear, gifted within a peculiar insight into the nature of reality. For him the ultimate reality is the eternal mind, so he believed that the task of imagination is to create shapes by which this reality can be revealed.
The Romantics agreed that their task was to find through the imagination some order which explains the world of appearances, for them this reality could be spiritual. They refused to accept the ideas of other men on trust or to sacrifice imagination to argument.
Keats had a passionate love for the visible world. To him, ultimate reality is to be found only in the imagination. He saw the imagination as a power which both creates and reveals, or rather reveals through creating. Keats accepted the works of the imagination not merely existing in their own right, but as having a relation to ultimate reality through the light which they shed on it. Through the imagination, Keats sought an absolute reality to which a door was opened by his appreciation of beauty through the senses. Through beauty he felt that he came into the presence of the ultimately real. The more intensely a beautiful object affected him, the more convinced he was that he passed beyond it to something else. The beauty of visible things carried Keats into ecstasy.
In conclusion, Coleridge, Blake, Wordsworth, Shelly and Keats were confident not only that the imagination was their most precious possession but that was somehow concerned with a supernatural order. They insisted that it reveals an important kind of truth; as it works it sees things to which the ordinary intelligence is blind, and that it is intimately connected with a special insight or perception or intuition.