By the Constitution the federal courts are given jurisdiction of all suits between two or more States, between a State and citizens of another State, between citizens of different States, between citizens of the same State claiming lands under grants of different States, and between a State, or the citizens thereof, and foreign States, citizens or subjects.
In this grant of jurisdiction the determining factor is not the nature of the matter litigated or the law involved, but the character of the parties to the suits. No question of federal concern, and no construction of a federal law or constitutional provision may be involved. The subjects to be determined may, and, indeed, usually, in this class of cases, depend wholly upon the interpretation and application of the laws of one or more of the States. The object in giving this jurisdiction to the federal courts is thus not the protection of federal rights, privileges, and inmiunities, but the provision of tribunals presumably more impartial than would be state tribunals when called upon to adjudicate between citizens of the State in which they are sitting and citizens of other States. This purpose is stated by Hamilton in No. LXXX of The Federalist. With reference to the clause of the Constitution providing that "the citizens of each State shall be entitled to all the privileges and immunities of citizens of the several States," he writes: "And if it be a just principle, that every government ought to possess the means of executing its own provisions, by its own authority, it will follow, that in order to the inviolable maintenance of that equality of privileges and immunities, to which the citizens of the Union will be entitled, the national judiciary ought to preside in all cases, in which one State or its citizens are opposed to another State or its citizens. To secure the full effect of so fundamental a provision against all evasion and subterfuge, it is necessary that its construction should be committed to that tribunal, which, having no local attachments, will be likely to be impartial, between the different States and their citizens, and which, owing its official existence to the Union, will never be likely to feel any bias inauspicious to the principles on which it is founded."
10 100 U. S. 257; 25 L. ed. 648.
That this exposition of Hamilton's correctly exhibits the aim sought by this provision is also shown by the debates in the Constitutional and State ratifying Conventions.11 treaties, or statutes of the United States shall otherwise require or provide, shall be regarded as rules of decision in trials at common law, in the courts of the United States, in cases where they apply." This provision has remained unaltered to the present day, and constitutes section 721 of the Revised Statutes,
In short, the theory is that the federal courts when thus called upon by reason of the diversity of the citizenship of the parties to construe and apply state law, are to consider themselves as ad hoc agents of the State, and, therefore, under an obligation to apply that law as they find it. This obligation was recognized in the 34th section of the original Judiciary Act of 1789, now section 721 of the Revised Statutes, which provides that: "The laws of the several States, except where the Constitution,
11 See, for example. Elliot's Debates, III, 533, 557, 566.