I did not realize Erie and Carolene were decided on the same day. Unlikely Beginnings of Modern Constitutional Thought by Louise Weinberg offers some analysis on that cool coincidence:

Since law without a relevant — “interested” — lawgiver is unconstitutional and no law at all, it is clear, after Erie, that American courts must choose. Erie holds, in effect, that a choice of law — the identification of the sovereign source of governance — is constitutionally required. A general rule is not the law of any identified government. The law chosen need not be that of the most relevant place, and need not be the law of the place of the most significant contact, or the place having the greatest interest. It need only be the law of an interested sovereign.

It is no doubt accidental that Carolene Products and Erie v. Tompkins were decided on the same day. But it is nevertheless the fact that a “rational basis” — the legitimate governmental interest of a relevant sovereign — is what is required for and authorizes judge-made law, under Erie, and statutory law, under Carolene. And once you grasp the interest-analytic metaphysics of Dick and Alaska Packers, and glimpse it in Erie as well, you can begin to see Carolene Products as an outcome and capstone of these unlikely beginnings.

I want to teach a class where the Court’s precedents are taught doctrinally, rather than sequentially (I tried it in one seminar, covering the top 300 cases of all time in two hours). Such a class would really illuminate these issues.

H/T Legal Theory Blog