"You are what you eat" is a dictum, and so is a law requiring you to curb your dog. A dictum is a formal pronouncement, a rule, or a statement that expresses a truth universally acknowledged.
Dictum dates from the 16th Century. It descended from a Latin word that means "something said." In contemporary use, it means more like "something that is officially said." If the principal of your school issues a dictum declaring "no jeans in school," it's time to go shopping.