Federalism:

Federations often employ the paradox of being a union of states, while still being states (or having aspects of statehood) in themselves. For example, James Madison (author of the US Constitution) wrote in Federalist Paper No. 39 that the US Constitution "is in strictness neither a national nor a federal constitution; but a composition of both. In its foundation, it is federal, not national; in the sources from which the ordinary powers of the Government are drawn, it is partly federal, and partly national..." This stems from the fact that states in the US maintain all sovereignty that they do not yield to the federation by their own consent. This was reaffirmed by the Tenth Amendment to the United States Constitution, which reserves all powers and rights that are not delegated to the Federal Government as left to the States and to the people.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What is the foundation for federalist paper no. 39?
it is federal, not national; in the sources from which the ordinary powers of the Government are drawn, it is partly federal, and partly national..