This book is a new edition of a short but fascinating treatise by Galen on causal theory. This text survives only in a Latin translation of the fourteenth century, and it is this which appears here. The volume also contains the first translation of the treatise into any modern language, and the first philosophical commentary thereon. The commentary ranges widely in Galen's voluminous oeuvre, and compares his views with those of other ancient theorists. The introduction deals in detail with Galen's life and work, with the background both philosophical and medieval to his causal theory, and with the history of the text itself.