Source: United States v. Lopez, 1995, US Supreme Court, majority opinion by Rehnquist
To uphold the Government's contentions here, we have to pile inference upon inference in a manner that would bid fair to convert congressional authority under the Commerce Clause to a general police power of the sort retained by the States. Admittedly, some of our prior cases have taken long steps down that road, giving great deference to congressional action. The broad language in these opinions has suggested the possibility of additional expansion, but we decline here to proceed any further. [1.] To do so would require us to conclude that the Constitution's enumeration of powers does not presuppose something not enumerated, and that there never will be a distinction between what is truly national and what is truly local. This we are unwilling to do.
1. This superlative answer and its use of logic especially helped me, but doesn't duplicate this; I still wouldn't know how to parse the bold text until after determining that the negatives in the bold text mustn't be cancelled. Why not though? How can you discover or foreknow so?