In order for a form to be realised, one needs to have suitable matter.
Aristotle provides the general theoretical framework for this enterprise in his In this work, Aristotle sets out the conceptual apparatus for his analysis, provides definitions of his fundamental concepts, and argues for specific theses about motion, causation, place and time, and establishes in bk.
Hence the overlap between the two disciplines, which often verges on inseparability. This means that when an entity moves or is at rest according to its nature reference to its nature may serve as an explanation of the event.
We have to describe how—to what extent, through what other processes, and due to what agency—the preconditions for the process of change or of being at rest are present, but once we have provided an account of these preconditions, we have given a complete account of the process.
As a rule there is a collaboration between these causes: matter provides the potentialities which are actualised by the form.
Accordingly, these causally relevant entities give rise to a hierarchic structure of explanation.