 Aristotle further elaborates on causality in the rest of Physics II and in Parts of Animals I. Aristotle explores the systematic interrelations among the four modes of causality and argues for the explanatory priority of the final cause. In so doing Aristotle not only expands on his theory of causality; he also builds explanatory principles that are specific to the study of nature. Aristotle considers these principles an indispensable theoretical framework for a successful investigation of the natural world. Both Physics II and Parts of Animals have a foundational character. More directly, Aristotle expects the student of nature to have mastered these principles before engaging in the investigation of any aspect of the natural world.