The history of this course of change in Roman law is exceedingly instructive. At the earliest dawn of the jurisprudence, the term in use for a Contract was one which is very familiar to the students of historical Latinity. It was _nexum_, and the parties to the contract were said to be _nexi_, expressions which must be carefully attended to on account of the singular durableness of the metaphor on which they are founded. The notion that persons under a contractual engagement are connected together by a strong _bond_ or _chain_, continued till the last to influence the Roman jurisprudence of Contract; and flowing thence it has mixed itself with modern ideas. What then was involved in this nexum or bond? A definition which has descended to us from one of the Latin antiquarians describes _nexum_ as _omne quod geritur per æs et libram_, "every transaction with the copper and the balance," and these words have occasioned a good deal of perplexity. The copper and the balance are the well-known accompaniments of the Mancipation, the ancient solemnity described in a former chapter, by which the right of ownership in the highest form of Roman Property was transferred from one person to another. Mancipation was a _conveyance_, and hence has arisen the difficulty, for the definition thus cited appears to confound Contracts and Conveyances, which in the philosophy of jurisprudence are not simply kept apart, but are actually opposed to each other. The _jus in re_, right _in rem_, right "availing against all the world," or Proprietary Right, is sharply distinguished by the analyst of mature jurisprudence from the _jus ad rem_, right _in personam_, right "availing a single individual or group," or obligation. Now Conveyances transfer Proprietary Rights, Contracts create Obligations--how then can the two be included under the same name or same general conception? This, like many similar embarrassments, has been occasioned by the error of ascribing to the mental condition of an unformed society a faculty which pre-eminently belongs to an advanced stage of intellectual development, the faculty of distinguishing in speculation ideas which are blended in practice. We have indications not to be mistaken of a state of social affairs in which Conveyances and Contracts were practically confounded; nor did the discrepance of the conceptions become perceptible till men had begun to adopt a distinct practice in contracting and conveying.