Under modern law, federal legislation is subject to “rational basis review” under the doctrinal rubric of “substantive due process.” That construction of the Fifth Amendment’s Due Process of Law Clause is notoriously difficult to justify as a matter of original constitutional meaning. Something functionally very similar to substantive due process, however, is easily justifiable as a matter of original constitutional meaning once one understands that the Constitution, for interpretative purposes, is best seen as a kind of fiduciary instrument. Fiduciary instruments operate against a background of legal norms that notably include a duty of care on the part of agents. All federal actors under the Constitution exercise delegated authority (from “We the People”) as agents, and thus all federal actors under the Constitution are bound by a duty of care. This duty has much affinity with the business judgment rule of corporate law, in that the scope of the duty of federal actors, as gleaned from eighteenth-century agency and corporate law, probably does not exceed avoidance of gross negligence. This Article examines the contours of this duty of care that forms part of the background of every constitutional grant of power.