By the obligation of a contract, we should understand the Constitution to mean, the duty of performing a legal agreement.  If the contract be lawful, the party is bound to perform it.  But bound by what?  What is it that binds him?  And this leads us to what we regard as a principal fallacy in the argument on the other side.  That argument supposes, and insists, that the whole obligation of a contract has its origin in the municipal law.  This position we controvert.  We do not say that it is that obligation which springs from conscience merely; but we deny that it is only such as springs from the particular law of the place where the contract is made.  It must be a lawful contract, doubtless; that is, permitted and allowed; because society has a right to prohibit all such contracts, as well as all such actions, as it deems to be mischievous or injurious.  But if the contract be such as the law of society tolerates, in other words, if it be lawful, then we say, the duty of performing it springs from universal law.  And this is the concurrent sense of all the writers of authority.

The duty of performing promises is thus shown to rest on universal law; and if, departing from this well-established principle, we now follow the teachers who instruct us that the obligation of a contract has its origin in the law of a particular State, and is in all cases what that law makes it, and no more, and no less, we shall probably find ourselves involved in inextricable difficulties.  A man promises, for a valuable consideration, to pay money in New York.  Is the obligation of that contract created by the laws of that State, or does it subsist independent of those laws?  We contend that the obligation of a contract, that is, the duty of performing it, is not created by the law of the particular place where it is made, and dependent on that law for its existence; but that it may subsist, and does subsist, without that law, and independent of it.  The obligation is in the contract itself, in the assent of the parties, and in the sanction of universal law.  This is the doctrine of Grotius, Vattel, Burlamaqui, Pothier, and Rutherforth.  The contract, doubtless, is necessarily to be enforced by the municipal law of the place where performance is demanded.  The municipal law acts on the contract after it is made, to compel its execution, or give damages for its violation.  But this is a very different thing from the same law being the origin or fountain of the contract.

Let us illustrate this matter by an example.  Two persons contract together in New York for the delivery, by one to the other, of a domestic animal, a utensil of husbandry, or a weapon of war.  This is a lawful contract, and, while the parties remain in New York, it is to be enforced by the laws of that State.  But if they remove with the article to Pennsylvania or Maryland, there a new law comes to act upon the contract, and to apply other remedies if it be broken.  Thus far the remedies are furnished