Criminal law scholars have pined for a substantive constitutional criminal law ever since Henry Hart and Herbert Packer first embraced the notion in the late 1950s and early 1960s. To this day, scholars continue to search for a theory fhat giv:es content to, in Hart\u27s words, the unmistakable indications that the Constitution means something definite and spμiething serious when it speaks of \u27crime.\u27 To their dismay, the Supreme Court has - with two exceptions - seemingly resisted the notion. The two exceptions are familiar. First came the 1957 case of Lambert v. California, in which the Court came as close as it ever has to constitutionalizing a mens rea requirement as fundamental to the just imposition of a criminal sanction. Lam...