Article 1, Section 9, Clause 3
St. George Tucker, Blackstone's Commentaries 1:App. 292--931803
Bills of attainder are legislative acts passed for the special purpose of attainting particular individuals of treason, or felony, or to inflict pains and penalties beyond, or contrary to the common law. They are state-engines of oppression in the last resort, and of the most powerful and extensive operation, reaching to the absent and the dead, as well as to the present and the living. They supply the want of legal forms, legal evidence, and of every other barrier which the laws provide against tyranny and injustice in ordinary cases: being a legislative declaration of the guilt of the party, without trial, without a hearing, and often without the examination of witnesses, and subjecting his person to condign punishment, and his estate to confiscation and forfeiture. Instances of their application to these nefarious purposes occur in almost every page of the English history for a very considerable period: and very few reigns have passed in which the power has not been exercised, though, to the honour of the nation, I believe, no instance of the kind has occurred for more than half a century.
In May, 1778, an act passed in Virginia, to attaint one Josiah Philips, unless he should render himself to justice, within a limited time: he was taken, after the time had expired, and was brought before the general court to receive sentence of execution pursuant to the directions of the act. But the court refused to pass the sentence, and he was put upon his trial, according to the ordinary course of law. . . . This is a decisive proof of the importance of the separation of the powers of government, and of the independence of the judiciary; a dependent judiciary might have executed the law, whilst they execrated the principles upon which it was founded.
If any thing yet more formidable, or more odious than a bill of attainder can be found in the catalogue of state-enginery, it is what the constitution prohibits in the same clause, by the name of ex post facto laws: whereby an action indifferent in itself, and not prohibited by any law at the time it is committed, is declared by the legislature to have been a crime, and punishment in consequence thereof, is inflicted on the person committing it. Happily, for the people of Virginia, I can not cite any case of an ex post facto law, (according to this definition, which I have borrowed from Judge Blackstone,) that has been made in this commonwealth, nor have I heard of any such, in any other of the United States, that I recollect.
The Founders' Constitution
Volume 3, Article 1, Section 9, Clause 3, Document 12
The University of Chicago Press
Tucker, St. George. Blackstone's Commentaries: With Notes of Reference to the Constitution and Laws of the Federal Government of the United States and of the Commonwealth of Virginia. 5 vols. Philadelphia, 1803. Reprint. South Hackensack, N.J.: Rothman Reprints, 1969.
Easy to print version.