In Chapter XIII of Bigraphia Literaria, Coleridge writes:

“The imagination then I consider either as primary, or secondary”.

According to Coleridge, Imagination has two forms; primary and secondary. Primary imagination is merely the power of receiving impressions of the external world through the senses, the power of perceiving the objects of sense, both in their parts and as a whole. It is a spontaneous act of the mind; the human mind receives impressions and sensations from the outside world, unconsciously and involuntarily, imposes some sort of order on those impressions, reduces them to shape and size, so that the mind is able to form a clear image of the outside world. In this way clear and coherent perception becomes possible.

The primary imagination is universal, it is possessed by all. The secondary imagination may be possessed by others also, but it is the peculiar and typical trait of the artist. It is the secondary imagination which makes artistic creation possible. Secondary imagination is more active and conscious; it requires an effort of the will, volition and conscious effort. It works upon its raw material that are the sensations and impressions supplied to it by the primary imagination. By an effort of the will and the intellect the secondary imagination selects and orders the raw material and re-shapes and re-models it into objects of beauty. It is ‘esemplastic’, i.e. “a shaping and modifying power”. Its ‘plastic stress’ re-shapes objects of the external world and steeps them with a glory and dream that never was on sea and land. It is an active agent which, “dissolves, diffuses, dissipates, in order to create”.

This secondary imagination is at the root of all poetic activity. It is the power which harmonizes and reconciles opposites. Coleridge calls it a magical, synthetic power. This unifying power is best seen in the fact that it synthesizes or fuses the various faculties of the soul – perception, intellect, will, emotion – and fuses the internal with the external, the subjective with the objective, the human mind with external nature, the spiritual with the physical. Through this unifying power nature is colored by the soul of the poet, and soul of the poet is steeped in nature. ‘The identity’ which the poet discovers in man and nature results from the synthesizing activity of the secondary imagination.

The primary and secondary imaginations do not differ from each other in kind. The difference between them is one of degree. The secondary imagination is more active, more a result of volition, more conscious and more voluntary than the primary one. The primary imagination is universal while the secondary is a peculiar privilege enjoyed by the artist.

Imagination and fancy, however, differs in kind. Fancy is not a creative power at all. It only combines what is perceives into beautiful shapes, but like the imagination it does not fuse and unify. The difference between the two is the same as the difference between a mechanical mixture and a chemical compound. In a mechanical mixture a number of ingredients are brought together. They are mixed up, but they do not lose their individual properties. In a chemical compound, the different ingredients combine to form something new. The different ingredients no longer exist as separate identities. They lose their respective properties and fuse together to cerate something new and entirely different. A compound is an act of creation; while a mixture is merely a bringing together of a number of separate elements.

Thus imagination creates new shapes and forms of beauty by fusing and unifying the different impressions it receive from the external world. Fancy is not creative. It is a kind of memory; it randomly brings together images, and even when brought together, they continue to retain their separate and individual properties. They receive no coloring or modification from the mind. It is merely mechanical juxtaposition and not a chemical fusion. Coleridge explains the point by quoting two passages from Shakespeare's Venus and Adonis. The following lines from this poem serve to illustrate Fancy:

Full gently now she takes him by the hand.
A lily prisoned in a goal of snow
Or ivory in an alabaster band
So white a friend engirds so white a foe.

In these line images are drawn from memory, but they do not interpenetrate into one another. The following lines from the same poem illustrate the power and function of Imagination:

Look! How a bright star shooteth from the sky
So glides he in the night from Venus’ eye.

For Coleridge, Fancy is the drapery of poetic genius but imagination is its very soul which forms all into one graceful and intelligent whole.

Coleridge owed his interest in the study of imagination to Wordsworth. But Wordsworth was interested only in the practice of poetry and he considered only the impact of imagination on poetry; Coleridge on the other hand, is interested in the theory of imagination. He is the first critic to study the nature of imagination and examine its role in creative activity. Secondly, while Wordsworth uses Fancy and Imagination almost as synonyms, Coleridge is the first critic to distinguish between them and define their respective roles. Thirdly, Wordsworth does not distinguish between primary and secondary imagination. Coleridge’s treatment of the subject is, on the whole, characterized by greater depth, penetration and philosophical subtlety. It is his unique contribution to literary theory.