Letter on the Deaf and Dumb, for the Use of those who hear and speak (French: Lettre sur les sourds et muets a l'usage de ceux qui entendent et qui parlent) is a work by Denis Diderot containing a psychological investigation on the deaf-mute. It was published in 1751. It was meant to be a companion volume to Diderot's Letter on the Blind. Source: Wikipedia (en)