What are the rules/laws of magic?

I read a quote that said "He can wield it only so long as he strictly conforms to the rules of his art, or to what may be called the laws of nature as conceived by him. To neglect these rules, to break these laws in the smallest particular is to incur failure, and may even expose the unskillful practitioner himself to the utmost peril."

What rules/law is this guy specifically referring to?