Ellingburg v. United States · ¶26

Ex post facto laws lack legitimacy because laws must precede the actions that they govern. Laws regulating actions after the fact “deprive citizens of notice and fair warning and are, therefore, an affront to man's `reason and freewill.' ” Peugh v. United States, 569 U. S. 530, 561 (2013) (Thomas, J., dissenting) (quoting 1 W. Blackstone, Commentaries on the Laws of England 39 (1765) (Blackstone)). As Blackstone explained, “it is impossible that the party could foresee” that his actions would become unlawful if they were lawful when he took them. Id., at 46. When a man had no notice that his actions were unlawful, “all punishment for not abstaining must of consequence be cruel and unjust.” Ibid. An ex post facto law is even “more unreasonable,” Blackstone thought, than the reviled practice of the Roman emperor “Caligula, who . . . wrote his laws in a very small character, and hung them…

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