Why do organisms reproduce? Why do birds have wings? Why do neither snakes nor stars have feet? And why do most of the hoofed life-bearing animals have horns (but not all of them)?

For Aristotle, questions such as these go to the heart of natural philosophy, which is the study of the coming to be and presence of beings that have their own internal principle of change and rest. Throughout his lifetime, Aristotle was deeply committed to investigating and explaining natural phenomena, which is reflected all through the surviving treatises on natural philosophy. Among these, Aristotle's Physica is most fundamental. In this treatise, Aristotle lays out the general theoretical framework for his natural philosophy, defining notions such as nature, motion, causation, place, and time. In the other treatises, Aristotle explores more specific problems related to the study of natural beings, such as coming to be and passing away (in De Generatione et Corruptione), the nature and motion of the elements (in De Generatione et Corruptione and the second part of the De Caelo), the motions and features of the heavenly bodies (in the first part of the De Caelo), atmospheric causes and changes (in the Meteorologica), the notion of soul and its dependence on natural bodies (in De Anima), and finally, the causes of the coming to be and presence of living beings and of their parts and motions (in the biological works).