 On the Parts of Animals, book I (PA I) begins by outlining its purpose, which is to establish a set of standards for judging natural investigations (639a15). Its five chapters pursue this purpose, discussing the appropriate level of generality for such studies, the modes of causality and of necessity to be used in biological explanations, the relation of form to matter in living things, the proper method of logical division for this subject matter, the means of identifying kinds and their activities at the proper level of abstraction, and much more. Two sorts of evidence support the conclusion that this book is intended to deal with problems and questions that arise in the application of Aristotle’s general philosophy of science, found in the Posterior Analytics, to his theoretical investigation of living nature.