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.

But this account of the matter is at least questionable.  So far as the use of words goes, I am not aware that consideration is distinctly called cause before the reign of Elizabeth; in the earlier reports it always appears as quid pro quo.  Its first appearance, so far as I know, is in Fleta’s account of the action of debt, 1 and although I am inclined to believe that Fleta’s statement is not to be trusted, a careful consideration of the chronological order of the cases in the Year Books will show, I think, that the doctrine was fully developed in debt before any mention of it in equity can be found.  One of the earliest [254] references to what a promisor was to have for his undertaking was in the action of assumpsit. 1 But the doctrine certainly did not originate there.  The first mention of consideration in connection with equity which I have seen is in the form of quid pro quo, 2 and occurs after the requirement had been thoroughly established in debt. 3

The single fact that a consideration was never required for contracts under seal, unless Fleta is to be trusted against the great weight of nearly contemporaneous evidence, goes far to show that the rule cannot have originated on grounds of policy as a rule of substantive law.  And conversely, the coincidence of the doctrine with a peculiar mode of procedure points very strongly to the probability that the peculiar requirement and the peculiar procedure were connected.  It will throw light on the question to put together a few undisputed facts, and to consider what consequences naturally followed.  It will therefore be desirable to examine the action of debt a little further.  But it is only fair to admit, at the outset, that I offer the explanation which follows with great hesitation, and, I think, with a full appreciation of the objections which might be urged.