"For Hearth and Altar," an exhibition of 125 African terra cotta pots made mostly during the 20th century, is the Art Institute of Chicago’s most stimulating, challenging show in memory. It takes us back to the origins of civilization and raises fundamental questions about the nature of art. Severe and unfamiliar, the work delivers no lazy sensual delights. Differences among the pieces are subtle. Viewers must look -- and think -- carefully to get the best from this work.