Michael Greve on the Health Care Case and the Meaning of "Proper"
“Necessary” is a means-ends test: legislation must be necessary (convenient, useful) to a constitutionally provided-for end. “Proper,” in contrast, can’t be a means-ends test, at least not exclusively: if it were, it would be swallowed by ”necessary” and judicial deference canons. It’s best read (to my mind) as shorthand for the proposition (Marshall’s proposition) that legislation must be consistent with the letter and the spirit of the Constitution—not its ghost but its structural principles, including principles that (unlike the principle of limited and enumerated powers) aren’t immediately obvious.
UPDATE: Ilya Somin comments here.