The prohibition against its making any ex post facto laws was introduced for greater caution, and very probably arose from the knowledge that the Parliament of Great Britain claimed and exercised a power to pass such laws under the denomination of bills of attainder or bills of pains and penalties, the first inflicting capital and the other less punishment. These acts were legislative judgments and an exercise of judicial power. Sometimes they respected the crime by declaring acts to be treason which were not treason when committed; at other times they violated the rules of evidence (to supply a deficiency of legal proof) by admitting one witness when the existing law required two, by receiving evidence without oath or the oath of the wife against the husband, or other testimony which the courts of justice would not admit; at other times they inflicted punishments where the party was not by law liable to any punishment, and in other cases they inflicted greater punishment than the law annexed to the offense. The ground for the exercise of such legislative power was this, that the safety of the kingdom depended on the death or other punishment of the offender, as if traitors, when discovered, could be so formidable or the government so insecure! With very few exceptions, the advocates of such laws were stimulated by ambition or personal resentment and vindictive malice. To prevent such and similar, acts of violence and injustice, I believe, the federal and state legislatures were prohibited from passing any bill of attainder or any ex post facto law.
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I shall endeavor to show what law is to be considered an ex post facto law within the words and meaning of the prohibition in the federal Constitution. The prohibition “that no state shall pass any ex post facto law” necessarily requires some explanation, for naked and without explanation it is unintelligible and means nothing. Literally, it is only that a law shall not be passed concerning and after the fact or thing done or action committed. I would ask, what fact, of what nature, or kind, and by whom done? That Charles I, King of England, was beheaded, that Oliver Cromwell was Protector of England, that Louis XVI, late King of France, was guillotined, are all facts that have happened, but it would be nonsense to suppose that the states were prohibited from making any law after either of these events and with reference thereto. The prohibition in the letter is not to pass any law concerning and after the fact, but the plain and obvious meaning and intention of the prohibition is this – that the legislatures of the several states shall not pass laws after a fact done by a subject or citizen which shall have relation to such fact and shall punish him for having done it. The prohibition, considered in this light, is an additional bulwark in favor of the personal security of the subject, to protect his person from punishment by legislative acts having a retrospective operation. I do not think it was inserted to secure the citizen in his private rights of either property or contracts. The prohibitions not to make anything but gold and silver coin a tender in payment of debts and not to pass any law impairing the obligation of contracts were inserted to secure private rights, but the restriction not to pass any ex post facto law was to secure the person of the subject from injury or punishment in consequence of such law. If the prohibition against making ex post facto laws was intended to secure personal rights from being affected, or injured by such laws, and the prohibition is sufficiently extensive for that object, the other restraints I have enumerated were unnecessary, and therefore improper, for both of them are retrospective.I will state what laws I consider ex post facto laws within the words and the intent of the prohibition. 1st. Every law that makes an action done before the passing of the law and which was innocent when done, criminal and punishes such action. 2d. Every law that aggravates a crime, or makes it greater than it was when committed. 3rd. Every law that changes the punishment, and inflicts a greater punishment than the law annexed to the crime, when committed. 4th. Every law that alters the legal rules of evidence and receives less or different testimony than the law required at the time of the commission of the offense in order to convict the offender.Page 3 U. S. 391All these and similar laws are manifestly unjust and oppressive. In my opinion, the true distinction is between ex post facto laws and retrospective laws. Every ex post facto law must necessarily be retrospective, but every retrospective law is not an ex post facto law. The former only are prohibited. Every law that takes away or impairs rights vested agreeably to existing laws is retrospective, and is generally unjust and may be oppressive, and it is a good general rule that a law should have no retrospect; but there are cases in which laws may justly, and for the benefit of the community and also of individuals, relate to a time antecedent to their commencement, as statutes of oblivion or of pardon. They are certainly retrospective, and literally both concerning and after the facts committed. But I do not consider any law ex post facto within the prohibition that mollifies the rigor of the criminal law, but only those that create or aggravate the crime or increase the punishment or change the rules of evidence for the purpose of conviction. Every law that is to have an operation before the making thereof, as to commence at an antecedent time or to save time from the statute of limitations or to excuse acts which were unlawful, and before committed, and the like, is retrospective. But such laws may be proper or necessary, as the case may be. There is a great and apparent difference between making an unlawful act lawful and the making an innocent action criminal and punishing it as a crime. The expressions “ex post facto laws” are technical; they had been in use long before the Revolution, and had acquired an appropriate meaning, by legislators, lawyers, and authors.The celebrated and judicious Sir William Blackstone, in his commentaries, considers an ex post facto law precisely in the same light I have done. His opinion is confirmed by his successor, Mr. Wooddeson, and by the author of the Federalist, who I esteem superior to both for his extensive and accurate knowledge of the true principles of government.I also rely greatly on the definition, or explanation of ex post facto laws as given by the Conventions of Massachusetts, Maryland, and North Carolina in their several constitutions or forms of government.