Oath, a solemn act by which one calls God to witness the truth of an affirmation or the sincerity of a promise. In all times and among all nations men have agreed in reposing singular trust in declarations made under such a sanction. In primitive and in all purer states of society, solemn oaths, it would seem, have been universally taken in the name of superior beings. Among the Jews, the Greeks, and the Romans there came to be a familiar distinction between their greater and their lesser oaths. The same is probably true of other nations. The less solemn forms of adjuration included oaths by sacred objects, or by things peculiarly dear to those who employed them. Thus, the Jews swore by Jerusalem and by the temple; the Greeks as well as the Romans by the souls of the dead, by the ashes of their fathers, by their life or the lives of their friends, by their heads, and their right hands. These forms had their origin partly too, perhaps, in the custom of touching, during the recital of the usual formula, some object sacred to or suggestive of the divinity invoked; so that, as during the administration of the oath the swearer laid his hand upon a crucifix as a sacred symbol, or touched the altar while he swore by the God in whose honor it was raised, he came at last to swear not by the divinity, but by the altar or the "good rood" itself.
When the Jew took his most solemn oath, he laid his hand upon the book of the law and swore by the God of Israel; but the ordinary oaths were by heaven, the altar, or the temple. Public oaths were administered in Athens in the names of Jupiter, Neptune, and Minerva; purgatory oaths were taken in the names of Jupiter, Neptune, and Themis; and judges swore by Jupiter, Ceres, and Helios. Numa commanded the Romans to swear by Fides. After the murder of Cfesar, the senate decreed that the citizens should swear by his genius. It was subsequently common to swear by the majesty or by the life or welfare of the emperor. The ancient Scandinavians and Germans swore by their gods. Among both races it was customary, while repeating the oath, to rest the hand on some special object. This was sometimes significant of the god addressed, and sometimes reminded the swearer of the punishment which followed perjury. The Scandinavians touched a bloody ring held by the priest. The Germans swore by their swords or beards. In early Christian times oaths were administered in chapels and other holy places, at the altars, which for the occasion were rendered more sacred by placing upon them, holy relics. In modern times the Germans have sworn by God, and sometimes also by the holy evangel.
Ko-man Catholics add an invocation of the saints. - It is sometimes laid down that in the definition of an oath two things are to be distinguished: 1, the invocation by which God is called to witness the truth of what is sworn; and 2, the imprecation by which God is called on to punish falsehood. But many writers of authority define an oath without any mention of imprecation. Thus Cicero speaks of an oath as an affirmation under the sanction of religion; and more explicitly Voet, in his " Commentaries upon the Pandects," writes that it is a religious affirmation of the truth, or an invocation of the name of God in witness of the truth. A Spanish jurist, Perez, defines an oath as an affirmation on any subject by the name of God and some sacred thing; and the author of Fleta as the affirmation or negation of some point confirmed by the attestation of a holy thing. Coke uses similar language to these. As a witness in taking an oath must be understood to make a formal and solemn appeal to the Supreme Being for the truth of the evidence which he is about to give, atheists, who deny the existence of a Supreme Being, and such infidels as profess a religion which does not bind them to speak the truth, could not by the common law be witnesses.
It was laid down in the leading and interesting case of Omichund v. Barker, that the competency of a witness in regard to his religious opinions should be tested by the questions whether he believed in a God, in the obligation of an oath, and in a future state of rewards and punishments. But it is not now required that the witness believe in future punishment. Generally a disbelief in a future state goes only to affect the credibility of the witness; but he will be admitted to testify under oath if only he believes in the existence of a God who will punish crime, it matters not whether in this life or in another. If, on being questioned, the witness reply that the usual form of the oath will be binding on his conscience, it would be irrelevant and unnecessary to examine him further as to his belief. What this may be is immaterial; for if he takes the oath, he is understood to assume its religious obligations, and subject himself to the legal penalties which are inflicted on perjury if he speak falsely. The English statute 17 and 18 Victoria, c. 25, permits those who from conscientious motives should be unwilling to take an oath, to make instead their solemn affirmation.
The same indulgence is granted by statutes in the United States; and in some of the states there are even further relaxations of the ancient rules.
(See Evidence.) The form of administering the oath is in every case that which most forcibly impresses on the swearer the obligation of the oath, or in other words is most binding on his conscience. Jews are sworn therefore on the Pentateuch, Mohammedans on the Koran, and those of other faiths with such forms or ceremonies as they may deem most binding. - Of the various kinds of oaths, the promissory, assertory, decisory, and calumniob causa are most frequently met with. Promissory oaths refer to future acts, like those taken by public officers for the guaranty of their faithful performance of official duties; or they attend the promise to execute some contract or undertaking. Assertory or affirmative oaths establish the certainty of a present or past fact. To this class belong the various forms of decisory oaths; for example, the voluntary, when, as in the civil law, one asserts the justice of his claim; or the necessary, when the judge calls in one of the contesting parties to swear to a matter doubtful to him, in order to assist his determination of the cause.
But the decisory oath by excellence is perhaps that in the civil law, which one party offers to the other, for the decision of the matter in dispute; that is to say, a party whose proof is defective may tender an oath to his adversary, offering to submit to whatever he shall thus declare touching the matter. If the fact in question lie particularly "within the knowledge of the latter, he must swear, or the plaintiff's general allegations will be regarded as proved. If the fact lie within the knowledge of both parties, he to whose oath it was referred may elect either to swear or to refer the matter back to the party who first tendered the oath. If he refuse to do either, the fact alleged is held to be confessed and proved. In the Roman system, when an action was brought, either party must, at the other's request, take an oath that he did not maintain or defend the cause calumnim causa; that is, for the mere sake of harassing his opponent. If the party challenged took the oath, the other could bring no action against him for reckless litigation.
Before Justinian this procedure was optional, but he made it a necessary preliminary to bringing or defending any action. - Extra-judicial oaths, that is to say, those which are taken without authority of law, as for instance by members of secret voluntary associations, are possessed of no legal force whatever, and their falsity subjects the party taking them to no penalties. The taking of such oaths, however, is sometimes made a statutory misdemeanor.