The theory of universals that was developed and upheld by Buddhist philosophers in premodern India since the sixth century CE is famous for its central claim that any judgment of a similarity between things is due not to anything substantially real that the things share, but to their difference from other things ("anyāpoha"). This book investigates the theory as it is presented in the "Apohasiddhi", a work written by the Buddhist monk and teacher Ratnakīrti in the eleventh century, during the last flowering of philosophical activity in Indian Buddhism.