  Natural philosophy has to know and teach the very essence of natural beings. First, it has to deal with their basic principles, such as matter and motion, which are not natural beings themselves. These principles of natural philosophy are discussed in Aristotle’s Physics. Moreover, natural philosophy has to deal with the accidents of natural beings understood through their causes. These are the subject of Aristotle’s other writings on nature, from On the Heavens to On the Soul (on Zabarella’s ideas on Physics, see Biard 2005).