This definition of substance is reexamined in Metaphysics, but is established in Categories. Aristotle himself makes it a point to state that to best understand something one must look to the first and most basic concept and that in this case is substance. In reexamining substance in Metaphysics there is a need to reestablish what substance is. Substance is the most basic of things in the world as since it cannot be predicable of something it is in the basis of succession that one should return to substance. Also, it is seen that substance is the first part and basis of a whole, and thus in considering the universe substance is first considered.
Substance can be either sensible or immovable.
The former is changeable and exists both eternal and perishable. It is that category that man as well as other animals and plants fall into. Since substance can exist apart and by themselves and in the end are things in their most basic form, it follows that all things have the same causes of movement and modification, since all that is being moved or modified is actually substance. The causes of this movement, Aristotle theorizes, will probably be the soul and the body. For that we look to Physics.