The 10 Categories

In a definition of Aristotle's ten categories this was said in regard to the first of the categories, substance.
"A substance of a thing is something essential about it; something unchangeable...after substance the other nine categories are accidents, things or qualities that are not essential to the things...without one exception, no substance can exist apart from accidents; in other words, all substances have changeable characteristics. The only exception to this is God. Since He is unchangeable, He is a substance with no accidents. All that God is is essential to Him; there is nothing accidental about Him."
"Material Logic" by Martin Cothran