§6. An _ex post facto_ law is literally a law made after an act is done, or which has effect upon an act after it is done. But it here means a law that makes punishable as a _crime_, an act which was not criminal when done. A law is also an _ex post facto_ law that increases the punishment of a crime after it has been committed. If, for example, a law should be passed by which a person, having previously killed another in lawfully defending his own life, should be made to suffer death, it would be an _ex post facto_ law, because killing in self-defense, before the passage of the law, was not punishable as a crime. Such also would be a law that should require all persons now charged with stealing, to be imprisoned for life, if found guilty; because the crime, when committed, was punishable by a shorter imprisonment. §7. The next prohibition is, "No capitation or other direct tax shall be laid, unless in proportion to the census or enumeration herein before directed to be taken." The words _capitation_ and _capital_ are from the Latin _caput_, the head, or poll. Hence a _capitation-tax_ or a _poll-tax_, is a tax upon each head or person. (Chap. VII. §4.) The above clause means, that poll-taxes, if laid, must be laid in conformity to article 1st, section 2d, clause 3d, of the constitution, which requires three-fifths of the slaves to be counted in apportioning taxes among the states according to population. §8. The next prohibition is, "No tax or duty shall be laid on articles exported from any state." Probably no law for taxing exports could be devised which would operate equally upon the interests of the different states. Or some states the principal product is cotton, rice, or tobacco; of others, grain; and of others, manufactures; and some of these products might not bear the same rates of duties as others. But though it were possible to devise a plan which would be equal in its operation, a majority of the representatives might be opposed to it. The representatives of the grain producing, and those of the planting states, might combine in imposing excessive taxes upon the productions of the manufacturing states. Or the manufacturing and the grain producing states might, with the same intent, combine against the planting states. §9. As it was the purpose of the framers of the constitution to make taxation, as nearly as possible, equal in the different states, by uniform duties; and as every necessary object of indirect taxation may be attained by duties on imports; duties on exports are properly prohibited. And to secure to all the states freedom and equality in trade, it is expressly provided in the same clause, that "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."