The substance of the plaintiff's claim as set forth in the writ of debt is that the defendant owes him so much and wrongfully withholds it. It does not matter, for a claim framed like that, how the defendant's duty arises. It is not confined to contract. It is satisfied if there is a duty to pay on any ground. It states a mere conclusion of law, not the facts upon which that conclusion is based, and from which the liability arises. The old German complaint was, in like manner, "A owes me so much." It was characteristic of the German procedure that the defendant could meet that complaint by answering, in an equally general form, that he did not owe the plaintiff. The plaintiff had to do more than simply allege a debt, if he would prevent the defendant from escaping in that way. In England, if the plaintiff had not something to show for his debt, the defendant's denial turned him out of court; and even if he had, he was liable to be defeated by the defendant's swearing with some of his friends to back him that he owed nothing. The chief reason why debt was supplanted for centuries by a later remedy, assumpsit, was the survival of this relic of early days. Finally, in England as in Germany, debt for the detention of money was the twin brother of the action brought for wrongfully withholding any other kind of chattel. The gist of the complaint in either case was the same. It seems strange that this crude product of the infancy of law should have any importance for us at the present time. Yet whenever we trace a leading doctrine of substantive law far enough back, we are very likely to find some forgotten circumstance of procedure at its source. Illustrations of this truth have been given already. The action of debt and the other actions of contract will furnish others. Debt throws most light upon the doctrine of consideration. Our law does not enforce every promise which a man may make. Promises made as ninety-nine promises out of a hundred are, by word of mouth or simple writing, are not binding unless there is a consideration for them. That is, as it is commonly explained, unless the promisee has either conferred a benefit on the promisor, or incurred a detriment, as the inducement to the promise. It has been thought that this rule was borrowed from Roman law by the Chancery, and, after undergoing some modification there, passed into the common law.