David, a classical critic of great eminence, was born at Stolpe, in Prussian Pomerania, in the year 1723. Of the early part of his studies little is known; but it appears that he was some time at Schlaff, from which he removed to Königsberg, where he met with the celebrated philosopher Kant. He afterwards repaired to Göttingen, to attend the lectures of Gesner, and to enlarge his knowledge of the Greek language. Some time after this period he formed an acquaintance with Ritter and Berger, whilst he resided at Wittenberg, where he continued about two years. His earliest production was a disputation De Galla Ruhnken, Placidia Augusta (1743), daughter of Theodosius, and the sister of Arcadius and Honorius. Under Berger he studied Roman antiquities and eloquence; under Ritter, jurisprudence and history. He relinquished the study of divinity, for which he was at first designed, and prevailed with his parents to allow him to transfer his residence to Leyden, where he arrived with recommendations to many of the learned. He pursued his studies with avidity and zeal, and accompanied Alberti in his visit to the Spa in the year 1750. Hemsterhuis wishing to attach him to Holland, urged him to persevere in the study of the law, as affording an additional chance of employment. This advice he thought proper to follow, and published a translation of some works of Theodorus Stephanus, and other Greek civilians. In the year 1755 he went to Paris, where Caperonier, who was at that time keeper of the king's library, received him kindly. He now formed an acquaintance with Dr Musgrave and Mr Tywhitt, who were there for the purpose of examining the manuscripts of Euripides. He had also formed the resolution of going to Spain; but Hemsterhuis recalled him, as he needed his assistance as lecturer in the Greek tongue. In 1755 Ruhnken took possession of his office, and read an excellent discourse De Gracia Artium et Doctrinarum Juventice. He was likewise useful to Ernesti in his edition of Callimachus; and in 1761 he succeeded Oudendorp as professor of history and of eloquence, delivering an inaugural oration De Doctore Umbriaco. About a year after this event Ruhnken was offered the chair of Gesner in the university of Göttingen. This offer he, however, declined; but, on his recommendation, the office was very worthily bestowed upon Heyne. In 1764 he married a beautiful Italian lady, who about six years afterwards lost both her speech and sight by a stroke of apoplexy. She had two daughters, one of whom was afterwards blind. The desire of Ruhnken to do Ernesti a favour made him turn his attention to the Memorabilia of Xenophon; and he was led to examine with particular attention the treatise of Longinus on the Sublime. Hemsterhuis died in 1767, and Ruhnken, then rector of the university, delivered a noble eulogium on his friend and patron. It was subsequently published under the title of Elogium Tiberii Hemsterhusi, 1768. In 1772 he prosecuted his new edition of Velleius Paterculus, and prepared a second edition of his Epistolae Criticae, and a collection of scholia on Plato. In the year 1768, he published a valuable tract De Vita et Scriptis Longini, in the form of an academical dissertation, to which he prefixed the name of one of his pupils. His Villeius Paterculus appeared in 1779, and in 1780 Homer's reputed hymn to Ceres. In 1786 he published the first part of Apuleius, which had been prepared by Oudendorp, and a new edition of his own Timaeus in 1789; and at the same time he collected and published the works of Muretus, in 5 vols. 8vo. Both the body and mind of Ruhnken were much weakened by domestic affliction; but he was in some measure relieved by the satisfaction he felt at the dedication of Homer by Wolf, although he was not of that writer's opinion, that the works of Homer were written by different authors. He sunk into a kind of stupor on the 14th of May 1798, which in two days put a period to his existence.

His knowledge and learning were immense, and he was unquestionably one of the very greatest scholars of the eighteenth century. He was lively, cheerful, and gay, even to an extreme. Many posthumous honours were conferred upon him, and a pension settled on his unfortunate widow. When Wyttenschau took possession of Ruhnken's chair, he delivered a discourse on the early age of Ruhnken, which he proposed as an example to the Batavian youth who made the belles lettres their study; and he afterwards wrote a Life of his master, Leyden,