The letter in chapter 13 is there to explain why Coleridge's description of the primary and secondary imagination in this chapter is so fragmentary and incomplete, or, as the letter puts it:

You have been obliged to omit so many links, from the necessity of compression, that what remains, looks (if I may recur to my former illustration) like the fragments of the winding steps of an old ruined tower.

The letter urges Coleridge not to include his full explanation in this volume. First, because it would run to about 100 pages, which would greatly increase the cost of the book to the reader. Secondly, because Coleridge's theory, the letter says, is so difficult to follow that it would lose most readers. Lastly, because it is "so abstruse a subject so abstrusely treated" that an ordinary reader—like the letter writer—would likely feel imposed-upon or annoyed.

Therefore, in this chapter, Coleridge offers only a cursory explanation of the two types of imagination. In short, he says, the primary imagination is the ordinary, everyday imagination of the average person that is "the living power and prime agent of all human perception." In other words, it simply repeats what it sees in nature. For example, if the primary imagination perceives a field of golden grain, this field of golden grain is all that it sees.

The secondary imagination, on the other hand, is what Coleridge considers to be rarer and more powerful: the imagination of the artistic genius (such as the imagination of men like Coleridge). This imagination "dissolves, diffuses, [and] dissipates [what it observes], in order to recreate." Thus, it is more creative than the primary imagination.

The best way to understand these two kinds of imaginations is to read Coleridge's Kubla Khan. The placid first stanza is the work of the primary imagination, while the second two stanzas—which are more powerful, dreamlike, and "demonic"—represent the secondary imagination.