J. D. Michaelis was trained for academic life under his father's eye. At Halle he was influenced, especially in philosophy, by Siegmund J. Baumgarten (1706-1757), the link between the old Pietism and J. S. Semler, while he cultivated his strong taste for history under Chancellor Ludwig. In 1739-1740 he qualified as university lecturer. One of his dissertations was a defence of the antiquity and divine authority of the vowel points in Hebrew. His scholarship still moved along the old traditional lines, and he was also much exercised by religious scruples, the conflict of an independent mind with that submission to authority at the expense of reason encouraged by the Lutheranism in which he had been trained.
A visit to England in 1741-1742 lifted him out of the narrow groove of his earlier education. In passing through the Netherlands he made the acquaintance of Albert Schultens, whose influence on his philological views became allpowerful a few years later. At Halle Michaelis felt himself out of place, and in 1745 he gladly accepted an invitation to Göttingen as Privatdozent. In 1746 he became professor extraordinarius, in 1750 ordinarius, and in Göttingen he remained till his death in 1791.
One of his works was a translation of four parts of Samuel Richardson's Clarissa; and translations of some of the then current English paraphrases on biblical books showed his sympathy with a school which attracted him by its freer air. His Oriental studies were reshaped by reading Schultens; for the Halle school, with all its learning, had no conception of the principles on which a fruitful connection between Biblical and Oriental learning could be established. His linguistic work indeed was always hampered by the lack of manuscript material, which is felt in his philological writings, e.g., in his valuable Supplementa to the Hebrew lexicons (1784-1792). He could not become such an Arabist as J. J. Reiske; and, though for many years the most famous teacher of Semitic languages in Europe, neither his grammatical nor his critical work has left a permanent mark, with the exception perhaps of his text-critical studies on the Peshitta.
His tastes were all for history, antiquities, and especially geography and natural science. He had in fact started his university course as a medicinae cultor, and in his autobiography he half regrets that he did not choose the medical profession. In geography he found a field hardly touched since Samuel Bochart, in whose footsteps he followed in the Spicilegium geographiae hebraeorum exterae post Bochartum (1769-1780); and to his impulse we owe the famous Royal Danish expedition to Arabia Felix (Yemen) conducted by Carsten Niebuhr and Peter Forsskål. In spite of his doctrinal writings -- which at the time made no little noise, so that his Compendium of Dogmatic (1760) was confiscated in Sweden, and the Knighthood of the North Star was afterwards given him in reparation -- it was the natural side of the Bible that really attracted him, and no man did more to introduce the method of studying Hebrew antiquity as an integral part of ancient Eastern life.
The personal character of Michaelis can be read between the lines of his autobiography with the aid of the other materials collected by J. M. Hassencamp (1743-1797) the editor (J. D. Michaelis Lebensbeschreibung, etc., 1793). The same volume contains a full list of his works. Besides those already mentioned it is sufficient to refer to his New Testament Introduction (the first edition, 1750, preceded the full development of his powers, and is a very different book from the later editions), his reprint of Robert Lowth's Praelectiones with important additions (1758-1762), his German translation of the Bible with notes (1773-1792), his Orientalische und exegetische Bibliothek (1775-1785) and Neue O. und E. Bib. (1786-1791), his Mosaisches Recht (1770-1771) (quite influenced by Montesquieu's L'esprit des lois of 1748) and his edition of Edmund Castell's LCXI con syriacum (1787-1788). His Litterarischer Briefwechsel (1794-1796) contains much that is interesting for the history of learning in his time.