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...
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