Oliver Sacks wrote this book in 1989. In his preface he writes that three years before he "knew nothing of the situation of the deaf". So this book is in many ways a chronicle of Sacks' own journey of discovery. Its main thesis is that the signing used by deaf people is indeed a fully fledged language with its own grammar and catalogue of nuances and styles. So, for example, if two or more deaf-signing people meet who have no spoken or written language in common, say American and Japanese (his example), within a day or two they are communicating fluently. The second half of the book, a chapter titled "The Revolution of the Deaf", is devoted to tracking a "revolution" at a university for deaf students who insist that the top academic positions should be occupied by deaf academics. Oliver Sacks champions this cause, becoming, to my mind, a touch uncomfortably evangelical: does he lose some objectivity? Nonetheless, "Seeing Voices" displays Sacks' trademark combination of compassion and deeply analytical insight.