Drawing on the work of Jeremy Bentham, we can forward a parity thesis concerning formal and substantive legal invalidity. Formal and substantive invalidity are, according to this thesis, traceable to the same source, namely, the sovereign's inability to adjust expectations to motivate obedience. The parity thesis, if defensible, has great appeal for positivists. Explaining why contradictory or contrary mandates yield invalidity is unproblematic. But providing an account of content-based invalidity invites the collapse of the separation between what the law is (...) and what the law ought to be. Grounding formal and substantive invalidity in a unified source allows us to avoid bringing in any additional apparatus that might compromise this separation. This essay fleshes out and defends the parity thesis. (shrink)