In the Biographia Literaria, Samuel Taylor Coleridge uses the phrase "Things identical must be convertible" to mean that if one identifies two things as being the same, each must be able to stand in place of the other. The example Coleridge gives to clarify this idea is that sleeping and being awake are not identical. When people are sleeping, they often dream and believe that they are awake. However, people who are awake do not believe that they are asleep. Both propositions would have to be true for sleeping and being awake to be convertible and therefore identical.

Coleridge uses this principle to illustrate what he regards as a key difference between prose and poetry. He opposes Wordsworth's view that there is no essential difference between poetry and prose except for the technical matters of meter and rhyme. Instead, he argues that there are "modes of expression, a construction, and an order of sentences" that occur in poetry but would be out of place in prose.