Usefulness to the master was in fact the end to which the changes in the law relating to servitude were directed. The slave might benefit his lord by a contract entered into with a third party, but could not make his condition worse.[569] The _dominus_ could sue on the contract, although the slave having no legal personality could conclude only a natural obligation (_naturalis obligatio_), but he was not liable for the losses. To protect third parties, however, and to give the necessary legal credit to this useful agency, the praetor gradually established a series of quasi-liabilities for the master, which were really in his interest; for without them slave-agency would have become impossible. Thus, if the master had countenanced the slave’s contract, he was liable (_actio quod jussu_); if the slave had embarked his _peculium_ in trade with the master’s knowledge, this property, though in strict law not his own, could be claimed by the creditors, after the slave’s debts to the master had been deducted (_actio tributoria_). Finally, any liability incurred by the _peculium_ could be recovered by creditors, the master’s right of deducting his own claims against it being preserved (_actio de peculio_), and any material advantage derived by the master from the contract of a slave was taken into consideration and the property of the _dominus_ made liable to that extent (_actio de in rem verso_).[570] The slave, in fact, as having no personality of his own, is the best of agents, and the theory of agency, which the law of Rome has bequeathed to us, is one of the most perfect and permanent results of her system of slavery. Apart from these relations to his master the slave was still ignored by law. He could not give evidence in court except under torture.[571] In case wrongs were done him, it was not he but his master that demanded reparation;[572] while his lord himself was the judge of the delicts which he had committed against himself or the household.[573] That for crimes against others the slave was tried by the ordinary process of criminal law was a concession to society rather than to the wrongdoer, and the sense of insecurity of the free population amidst their far more numerous dependants was expressed in the atrocious law that the murder of a Roman in his own house should be avenged by the death of the whole _familia_ that were sleeping beneath the roof at the moment of the commission of the crime.[574] The state itself owned slaves who were known as _servi publici_. Some were in the service of temples or of colleges of priests. Others were at the disposal of magistrates, such as the censors or aediles,[575] for the minor duties of attendance and police. Their agency in contracts was doubtless as useful to the state as that of private slaves was to individuals. Such an agent (_actor publicus_) was kept by the treasury for the acquisition of property,[576] which, as his _peculium_, fell under the _dominium_ of the state.