A letter to Henry Cline, Esq. on imperfect developements (sic) of the faculties mental and moral, as well as constitutional and organic; and on on the treatment of impediments of speech.

The first book on mental deficiency. Thelwall recognized that sensory deprivation could be a cause of apparent mental defect through his work with handicapped children. He established criteria for distinguishing between intellectual capability and performance. Digital facsimile from the Internet Archive at this link.