In prehistoric times, our ancestors began building shelters and planting crops in order to escape from nature's harsh realities. Today, we flee urban dangers for the safer, reconfigured world of suburban lawns and parks. According to geographer Yi-Fu Tuan, people have always sought to escape in one way or another, sometimes foolishly, often creatively and ingeniously. Glass-tower cities, suburbs, shopping malls, Disneyland-all are among the most recent monuments in our efforts to escape the constraints and uncertainties of life-ultimately, those imposed by nature. ''What cultural product,'' Tuan asks, ''is not escape?'' In his new book, the capstone of a celebrated career, Tuan shows that escapism is an inescapable component of human thought and culture.