Jean Fox, to whom, according to Voltaire, France was indebted for the science of medals, and Louis XIV. for one half of his cabinet, was born at Beauvais on the 24th of May 1632. At the age of three, he lost his father, and the charge of his education was undertaken by one of his maternal uncles. This uncle, who belonged to the magistracy, and who had destined him for his successor, died after bequeathing him his name and a part of his fortune; and being thus left to follow his own inclination, he relinquished the study of law for that of physic, and took a doctor's degree. He engaged in practice at Beauvais, but was accidentally drawn into a new department of study, which he prosecuted with great ardour during the remainder of his life. A farmer of that vicinity had found a great quantity of ancient coins, and Vaillant at first inspected them in a cursory manner; but perceiving that they bore a reference to events either forgotten or inaccurately detailed by historians, he was induced to resume their examination with deep attention, and he succeeded in explaining them with a facility which is generally the result of long experience. Having afterwards made a visit to Paris, he became known to Seguin, eminent for his numismatic skill, and astonished him by the extent of his knowledge. He was introduced to other men of learning; and the minister Colbert proposed to employ him in travelling to collect medals for the royal cabinet. He accepted of the offer, and after travelling into Italy, Greece, Egypt, and Persia, he returned with so many medals as made the king's cabinet superior to any in Europe. In one of his voyages, in 1674, the ship was taken by an Algerine corsair. After a captivity of nearly five months, he was permitted to return to France, and received at the same time twenty gold medals which had been taken from him. He embarked in a vessel bound for Marseille, and was carried with a favourable wind for two days, when another corsair appeared, which, in spite of all the sail they could make, bore down upon them within the reach of cannon shot. Vaillant, dreading the miseries of a fresh slavery, resolved at all events to secure the medals which he had recovered at Algiers, and therefore swallowed them. But a sudden change of the wind freed them from this adversary, and cast them upon the coast of Catalonia; and after expecting to run aground every moment, they at length fell among the sands at the mouth of the Rhone. He reached the shore in a skiff, but felt himself extremely incommoded with the medals he had swallowed, which might weigh altogether five or six ounces, and therefore did not pass like Scarborough waters. He had recourse to a couple of physicians, who were a little puzzled with the singularity of his case; however, nature relieved him from time to time, and he found himself in possession of the greatest part of his treasure when he arrived at Lyon. Among his collection was an Otho, valuable for its rarity. He was much caressed on his return; and when Louis XIV. gave a new form to the Academy of Inscriptions in 1701, Vaillant was first made associate, and then pensionary. In the course of his researches, he had twelve times visited Italy, and twice England and Holland. He was successively married to two sisters; having obtained a dispensation from the pope, upon condition of working for some time as a common labourer at St. Peter's church in Rome. He died on the 23rd of October 1706, in the seventy-fifth year of his age.

Vaillant possessed great knowledge and skill as an antiquary, and published various works, of which the following are the most important. Numismata Imperatorum Romanorum praestantiora, a Julio Cesare ad Posthumum et Tyrannos. Paris. 1674, 4to. Paris. 1694, 2 tom. 4to. Roma, 1743, 3 tom. 4to. This last edition contains additions and improvements by Baldini; and a supplement was published under the title of Jos. Khell ad Numismata Imperatorum Romanorum a Vaillantio edita Supplementum. Vienne, 1757, 4to. Seleucidarum Imperium: sive, Historia Regum Syriae, ad fidem Numismatum accommodata. Paris. 1681, 4to. Hag. Com. 1732, fol. Numismata Erea Imperatorum, Augustorum, et Cesarum, in Colonias, Municipiis, et Urbibus jure Latino donatis, ex omni modulo percussa. Paris. 1688, fol. Numismata Imperatorum, Augustorum, et Cesarum, a Populis Romanae ditionis Grace loquentibus ex omni modulo percussa. Paris. 1695, 4to. Amst. 1700, fol. Historia Ptolemaeorum Aegypti Regum, ad fidem Numismatum accommodata. Amst. 1701, fol. Nummi Antiqui Familiarum Romanarum, perpetuis interpretationibus illustrati. Amst. 1703, 2 part. fol. Arsacidarum Imperium: sive, Regum Parthorum Historia, ad fidem Numismatum accommodata. Achaemenidarum Imperium: sive, Regum Ponti, Bosphori, et Bithyniae Historia, ad fidem Numismatum accommodata. Paris. 1725, 2 tom. 4to. This posthumous work was published by Charles de Valois, of the Académie des Inscriptions.