Al-Hasan ar-Rassas(d. 1188 C.E.) was one of the most prominent representatives of the emerging reception of Basran Mu'tazilism in 12th century Yemen. He composed highly sophisticated works on ontology, causality and the specific theory of attributes that has become known as the theory of "states". He is therefore a rare case of a Mu'tazili thinker who left a comprehensive and systematised account of his natural philosophy. Jan Thiele's study makes extensive use of hitherto unexplored manuscripts, thereby providing new insight into a largely unknown chapter of Zaydi and Muslim doctrinal history. Focusing on the later Mu'tazila, this book conveys a nuanced understanding of diachronic developments in the school's teachings in order to refine a rather static perception that prevails in modern scholarship.