Clearly the government doesn't have any -- it's just a matter of what's on a list. Sure we can say that the more a substance is addictive, the more we want to

Clearly the government doesn't have any -- it's just a matter of what's on a list. Sure we can say that the more a substance is addictive, the more we want to put it into the category of "drug"; the more it impairs someone, the more it fits in our vague sense of what "drug" means. The author isn't saying "look, no necessary or sufficient condition exists, therefore checkmate". Also, I hope you agree that the word "definition" means something akin to "a statement of the exact meaning of a word" - and the more vague a thing's category, the less we have a definition of it.