A MAN WITHOUT WORDS By Susan Schaller. Foreword by Oliver Sacks. 203 pp. New York: Summit Books.

A human being without language would seem to be a 19th-century phenomenon; at least that's what people tried to tell Susan Schaller. But one day in the late 1970's Ms. Schaller, while working as a sign-language interpreter in Los Angeles, encountered a 27-year-old deaf Mexican man who seemed bright and curious but who, as she quickly discovered, had no language whatsoever. No sign language, no written or spoken Spanish or English. The man, whom the author calls Ildefonso (a pseudonym), was an illegal alien who had worked at a variety of jobs all over the United States but had somehow managed to get by without knowing how to add or subtract or even how to tell time.

Ms. Schaller, fascinated, was determined to make linguistic contact with him. She succeeded; the man suddenly connected "cat" -- the picture, the sign and the written word. And he was hungry for more. For her, Ildefonso's breakthrough was every bit as exciting as Helen Keller's discovery of water at the well.

In essence Ms. Schaller's book, "A Man Without Words," is a meditation on the wonders of language. Without language, there is no way to understand the passage of time. Ildefonso had no idea what a birthday was. In order to get to work on time he memorized how the face of the clock looked. Ms. Schaller began to realize how crucial language is in the organization of our inner selves, how it influences our perceptions about the world. To teach adjectives, the author began with colors. When she hit the color green Ildefonso was horrified. Eventually Ms. Schaller realized that, for Ildefonso, green represented the immigration officials who frequently captured him -- the color of their trucks and uniforms, even the green card he didn't have. Without language, the color came to symbolize all that was frightening. Without some language system, some explanation, history and geography cannot be comprehended unless one has lived every moment in time and traveled every foot of ground. There isn't even a way to illuminate the concepts of deafness and hearing.

Seven years later, Ms. Schaller tried to re-establish contact with her student. Convinced his was not a unique case, she searched for others like him as well. She discovered that several teachers had worked with deaf people who had no language, many of whom were from different cultures or who had astonishingly protective parents. She also pored over studies of so-called wild children, consulted treatises on language such as "The Man With the Shattered World" by A. R. Luria, and talked to the physician and writer Oliver Sacks, who urged her to continue her pursuit, and who ultimately wrote the foreword to this book.