The U.S. Constitution grants Congress the right "to regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes."  Wouldn't that necessarily imply that there is commerce that is not "among the several states"?  If not, wouldn't the clause read "to regulate commerce," without any specifications?  Wickard v. Filburn suggests that the federal government does not see it that way, of course, but it's still worth raising the point. (And we're not even raising the question of what the constitutional definition of "commerce" is.  When the people ratified the constitution, a great deal of business activities were not thought of as "commerce."

Of course, if we have a "living constitution," we can simply reinterpret the language to suit current needs. But if that's the case, we can reinterpret it again, however we want.

Finally, back to Wickard.  The Court ruled that a man growing his own wheat on his own land was "commerce among the several states."  They did so because New Deal regulations on crop growing could not function if Wickard could legally do what he was doing.   The Court, at the time argued that since Wickard's acts impaired a certain type of regulation of commerce among the states, his action could be regulated under the commece clause. But it might be that the precise language of the constitution puts limits on both what and how it may regulate even commerce that is certainly "among the states."