In 1975, Harvard philosopher Hilary Putnam published a paper called "The Meaning of Meaning, '" in which he challenged the orthodox view in the philosophies of language and mind. The article's Twin Earth thought experiment was simple, but the philosophical conclusions deriving from it -- about the nature of meaning and reference, of thought, of knowledge -- were shocking. Now, on the twentieth anniversary of the publication of paper, Andrew Pessin and Sandford Goldberg have collected the very best of the writing by the leading philosophers on the subject of Twin Earth. With a new introductory essay by Hilary Putnam, this volume acquaints novice philosophers with one of the most important debates in twentieth century philosophy and provides seasoned readers with a useful compendium of writing on essential questions in the philosophies of language and mind.