This provision, then is directed not so much against the penalty (for limitations upon penalties are found elsewhere in the constitution,) as against the mode of trial.  Or we may say that it is intended to prevent conviction without a trial; for in previous times legislative bodies had frequently punished political enemies without even the form of a trial, or without giving them an opportunity to be heard in their own defense, by passing against them bills of attainder.

[2] An ex post facto law is, literally, one which acts back upon a deed previously performed.  But as here intended, it means a law making worse such an act, either by declaring criminal that which was not so regarded in law when committed, or by increasing the penalty and applying it to the act previously performed.

But a law may be passed making better, in a sense, some previous act.  That is, an unforseen but imperative necessity may call for the doing of something which is not unlawful, but which needs, yet has not received, the sanction of law.  This act may afterwards be legalized by the legislature.

The things forbidden by this clause would, if permitted, render unsafe all those personal rights for the security of which the constitution was framed and the government founded.

Clause 4.—­Direct Taxes

No capitation or other direct tax shall be laid, unless in proportion to the census or enumeration hereinbefore directed to be taken.

This clause emphasizes the first sentence of clause three, section two, of this article.  It was intended to prevent the taxation of the two-fifths of the slaves not enumerated for representation, and was evidently inserted as a concession to the slave states.  But the abolition of slavery takes from the clause all force except that mentioned at the beginning of this paragraph.

No capitation tax (that is, so much per head) has ever been levied by the general government.

Clause 5.—­Duties on Exports.

No tax or duty shall be laid on articles exported from any state.

This was designed to prevent discrimination against any state or section.

Though the question has never been judicially determined, it is generally understood that since anything exported must be exported from some state (or territory), this clause prohibits all export duties.

Clause 6.—­Commercial Restrictions.

No preference shall be given by any regulation of commerce or revenue to the ports of one state over those of another; nor shall vessels bound to or from one state, be obliged to enter, clear or pay duties in another.

This provision has the same object in view as that which requires duties to be uniform—­the impartial treatment of the several states.  It shows, too, the fear felt by many that the general government might show partiality.