How to dissect/parse 'enumeration ... does not presuppose something not enumerated'?

Question

Source: United States v. Lopez, 1995, US Supreme Court, majority opinion by Rehnquist

The Court reasoned that if Congress could regulate something so far removed from commerce, then it could regulate anything, and since the Constitution clearly creates Congress as a body with enumerated powers, this could not be so. Rehnquist concluded:

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.

Sequences of negative words still hamstring me, so please explain and show all steps and thought processes. Do all negative words in the bolded phrase lie within each other's scope? I cancelled the two negatives, but then the rewrite appears trivially true which implies a mistake somewhere:

[2.] ... the Constitution's enumeration ... does not presuppose something not enumerated

If any document enumerates X, then the ability of X to be enumerated, must have been presupposed! The majority's rejection of 1 implies that 2 (a logically, albeit trivially, true statement) differs from 1.

Answer 1

The sentence is fairly obtuse (legalese is notorious for this), but you're misreading this when you assume it's a double negative. Let's break the sentence down a bit. "Presuppose something not enumerated" is a bit awkward; a less awkward way to say it is "implies that there's something that's not on the list."
But 'the Constitution's enumeration' doesn't presuppose that, so we have
"the existence of the list doesn't imply that there's something not on the list."
Now, "A doesn't imply B" means "it's possible for A to be true and B to be false,"
so this is "it's possible that there is a list and yet there is not anything that isn't on the list."

"There isn't anything that isn't on the list" is the same as "everything is on the list." The difference is important: The opposite of a "there exists" statement is a "for all" statement -- by saying that there isn't anything with some property, we're making a claim that everything has the opposite property. So, when we stick this new claim into our sentence, we get
"it's possible that there's a list and everything is on the list."

On the other hand, your sentence 2 is
"because there's a list, there exists something on the list" = "there must be something on the list."
2 is not equivalent to sentence 1, which says: it's possible that everything is on the list.
Sentence 1 says "this is a possible state of the world,"
sentence 2 says "this is the only possible state of the world."
You can't turn "A does not imply B exists" into "A implies Q exists" for any A, B, or Q --
doing that turns a statement saying "this is a possible"
into a statement saying "this is the only possible state of the world."