Nexum,therefore,which originally signified a Conveyance of property, came insensibly to denote a Contract also, and ultimately so constant became the association between this word and the notion of a Contract, that a special term, Mancipium or Mancipatio, had to be used for the purpose of designating the true nexum or transaction in which the property was really transferred. Contracts are therefore now severed from Conveyances, and the first stage in their history is accomplished, but still they are far enough from that epoch of their development when the promise of the contractor has a higher sacredness than the formalities with which it is coupled. In attempting to indicate the character of the changes passed through in this interval, it is necessary to trespass a little on a subject which lies properly beyond the range of these pages, the analysis of Agreement effected by the Roman jurisconsults. Of this analysis, the most beautiful monument of their sagacity, Ineed not say more than that it is based on the theoretical separation of the Obligation from the Convention or Pact. Bentham and Mr. Austin have laid down that the "two main essentials of a contract are these: first, a signification by the promising party of his intention to do the acts or to observe the forbearances which he promises to do or to observe. Secondly, a signification by the promisee that he expects the promising party will fulfil the proffered promise." This is virtually identical with the doctrine of the Roman lawyers, but then, in their view, the result of these "significations" was not a Contract, but a Convention or Pact. A Pact was the utmost product of the engagements of individuals agreeing among themselves, and it distinctly fell short of a Contract. Whether it ultimately became a Contract depended on the question whether the law annexed an Obligation to it. A Contract was a Pact (or Convention) plus an Obligation. So long as the Pact remained unclothed with the Obligation, it was called nude or naked.
What was an Obligation? It is defined by the Roman lawyers as "Juris vinculum, quo necessitate adstringimur alicujus solvendae rei." This definition connects the Obligation with the Nexum through the common metaphor on which they are founded, and shows us with much clearness the pedigree of a peculiar conception. The Obligation is the "bond" or "chain" with which the law joins together persons or groups of persons, in consequence of certain voluntary acts. The acts which have the effect of attracting an Obligation are chiefly those classed under the heads of Contract and Delict, of Agreement and Wrong; but a variety of other acts have a similar consequence which are not capable of being comprised in an exact classification. It is to be remarked, however, that the act does not draw to itself the Obligation in consequence of any moral necessity,. it is the law which annexes it in the plenitude of its power, a point the more necessary to be noted, because a different doctrine has sometimes been propounded by modern interpreters of the Civil Law who had moral or metaphysical theories of their own to support. The image of a vinculum juris colours and pervades every part of the Roman law of Contract and Delict. The law bound the parties together, and the chain could only be undone by the process called solutio, an expression still figurative, to which our word "payment" is only occasionally and incidentally equivalent. The consistency with which the figurative image was allowed to present itself, explains an otherwise puzzling peculiarity of Roman legal phraseology, the fact that "Obligation" signified rights as well as duties, the right, for example, to have a debt paid as well as the duty of paying it. The Romans kept in fact the entire picture of the "legal chain" before their eyes, and regarded one end of it no more and no less than the other.
In the developed Roman law, the Convention, as soon as it was completed, was, in almost all cases, at once crowned with the Obligation, and so became a Contract; and this was the result to which contract-law was surely tending. But for the purpose of this inquiry, we must attend particularly to the intermediate stage -- that in which something more than a perfect agreement was required to attract the Obligation. This epoch is synchronous with the period at which the famous Roman classification of Contracts into four sorts -- the Verbal, the Literal, the Real, and the Consensual had come into use, and during which these four orders of Contracts constituted the only descriptions of engagement which the law would enforce. The meaning of the fourfold distribution is readily understood as soon as we apprehend the theory which severed the Obligation from the Convention. Each class of contracts was in fact named from certain formalities which were required over and above the mere agreement of the contracting parties. In the Verbal Contract, as soon as the Convention was effected, a form of words had to be gone through before the vinculum juris was attached to it. In the Literal Contract, an entry in a ledger or tablebook had the effect of clothing the Convention with the Obligation, and the same result followed, in the case of the Real Contract, from the delivery of the Res or Thing which was the subject of the preliminary engagement. The contracting parties came, in short, to an understanding in each case; but, if they went no further, they were not obliged to one another, and could not compel performance or ask redress for a breach of faith. But let them comply with certain prescribed formalities, and the Contract was immediately complete, taking its name from the particular form which it had suited them to adopt. The exceptions to this practice will be noticed presently.
