In Humphry Clinker,

 the reader learns a great deal about the travels of Matthew Bramble, Lydia Melford, Jerry Melford, Tabitha Bramble, and Winifred Jenkins through their letters to their contemporaries.  However, we are never introduced to these people; the responses are not included as part of Tobias Smollett's text. 

Who are these people, and what can we learn about the book as a whole through them?  What do the letters tell us about both who writing and who is being written to?  Are we missing something by the omission of a response, for is a rebuttal not also part of the story?  By looking at the textual evidence for these mute characters, we the reader can learn more about how perceptions play out in the plot of the novel, how we should question the validity of each source, and what the personalities of the individual writers are.