Moses Mendelssohn (6 September 1729 – 4 January 1786) was a German Jewish philosopher to whose ideas the renaissance of European Jews, Haskalah (the Jewish Enlightenment), is indebted. He has been referred to as the father of Reform Judaism.

Born to a poor Jewish family in Dessau and originally destined for a rabbinical career, Mendelssohn educated himself in German thought and literature and from his writings on philosophy and religion came to be regarded as a leading cultural figure of his time by both Germans and Jews. He also established himself as an important figure in the Berlin textile industry, which was the foundation of his family's wealth. Moses Mendelssohn's descendants include the composers Fanny and Felix Mendelssohn and the founders of the Mendelssohn banking house.

Moses Mendelssohn was born in Dessau. His father's name was Mendel and he later took the surname Mendelssohn ("Mendel's son"). Mendel Dessau was a poor scribe — a writer of torah scrolls — and his son Moses in his boyhood developed curvature of the spine. His early education was cared for by his father and by the local rabbi, David Fränkel, who besides teaching him the Bible and Talmud, introduced to him the philosophy of Maimonides. Fränkel received a call to Berlin in 1743. A few months later Moses followed him.

A refugee Pole, Zamoscz, taught him mathematics, and a young Jewish physician taught him Latin. He was, however, mainly self-taught. He learned to spell and to philosophize at the same time (according to the historian Graetz). With his scanty earnings he bought a Latin copy of John Locke's An Essay Concerning Human Understanding, and mastered it with the aid of a Latin dictionary. He then made the acquaintance of Aaron Solomon Gumperz, who taught him basic French and English. In 1750, a wealthy silk merchant, Isaac Bernhard, appointed him to teach his children. Mendelssohn soon won the confidence of Bernhard, who made the young student successively his bookkeeper and his partner.

Either Gumperz or Hess introduced Mendelssohn to Gotthold Ephraim Lessing in 1754, who became one of his greatest friends. The story goes that the first time Mendelssohn met Lessing, they played chess; therefore, in Lessing's play Nathan the Wise Nathan and Saladin first meet during a game of chess.

The Berlin of the day – the day of Frederick the Great – was in a moral and intellectual ferment. Lessing had recently produced the drama Die Juden, whose moral was that a Jew can possess nobility of character. This notion was then generally ridiculed as untrue. Lessing found in Mendelssohn the realization of his dream. Within a few months of the same age, the two became brothers in intellectual and artistic camaraderie. Lessing also brought Mendelssohn to public attention for the first time: Mendelssohn had written an essay attacking Germans' neglect of their native philosophers (principally Gottfried Leibniz), and lent the manuscript to Lessing. Without consulting the author, Lessing published Mendelssohn's Philosophical Conversations (Philosophische Gespräche) anonymously in 1755. In the same year there appeared in Danzig (Gdańsk) an anonymous satire, Pope a Metaphysician (Pope ein Metaphysiker), which turned out to be the joint work of Lessing and Mendelssohn.

Mendelssohn became (1756 – 1759) the leading spirit of Friedrich Nicolai's important literary undertakings, the Bibliothek and the Literaturbriefe, and ran some risk (which Frederick's good nature mitigated) by criticizing the poems of the King of Prussia. In 1762 he married Fromet Guggenheim, who survived him by twenty-six years. In the year following his marriage Mendelssohn won the prize offered by the Berlin Academy for an essay on the application of mathematical proofs to metaphysics, On Evidence in the Metaphysical Sciences; among the competitors were Thomas Abbt and Immanuel Kant (who came second). In October 1763 the king granted Mendelssohn the privilege of Protected Jew (Schutzjude) – which assured his right to undisturbed residence in Berlin.

As a result of his correspondence with Abbt, Mendelssohn resolved to write on the Immortality of the Soul. Materialistic views were at the time rampant and fashionable, and faith in immortality was at a low ebb. At this favourable juncture appeared the Phädon oder über die Unsterblichkeit der Seele (Phädon or On the Immortality of Souls; 1767). Modelled on Plato's dialogue of the same name, Mendelssohn's work possessed some of the charm of its Greek exemplar and impressed the German world with its beauty and lucidity of style. The Phädon was an immediate success, and besides being often reprinted in German was speedily translated into nearly all the European languages, including English. The author was hailed as the "German Plato," or the "German Socrates"; royal and other aristocratic friends showered attentions on him, and it was said that "no stranger who came to Berlin failed to pay his personal respects to the German Socrates."

So far, Mendelssohn had devoted his talents to philosophy and criticism; now, however, an incident turned the current of his life in the direction of the cause of Judaism. In April 1763, Johann Kaspar Lavater, then a young theology student from Zurich, made a trip to Berlin, where he visited the already famous Jewish philosopher with some companions. They insisted on Mendelssohn telling them his views on Jesus and managed to get from him the statement, that, provided the historical Jesus had kept himself and his theology strictly within limits of orthodox Judaism, Mendelssohn "respected the morality of Jesus' character". Six years later, in October 1769, Lavater sent Mendelssohn his German translation of Charles Bonnet's essay on Christian Evidences, with a preface where he publicly challenged Mendelssohn to refute Bonnet or if he could not then to "do what wisdom, the love of truth and honesty must bid him, what a Socrates would have done if he had read the book and found it unanswerable". Mendelssohn answered in an open letter in December 1769: "Suppose there were living among my contemporaries a Confucius or a Solon, I could, according to the principles of my faith, love and admire the great man without falling into the ridiculous idea that I must convert a Solon or a Confucius." The ongoing public controversy cost Mendelssohn much time, energy and strength.

Lavater later described Mendelssohn in his book on physiognomy, "Physiognomische Fragmente zur Beförderung der Menschenkenntnis und Menschenliebe" (1775 - 1778), as "a companionable, brilliant soul, with piercing eyes, the body of an Aesop — a man of keen insight, exquisite taste and wide erudition [...] frank and open-hearted" — ending his public praise with the wish of Mendelssohn recognizing, "together with Plato and Moses... the crucified glory of Christ". When, in 1775 the Swiss - German Jews, faced with the threat of expulsion, turned to Mendelssohn and asked him to intervene on their behalf with "his friend" Lavater, Lavater, after receiving Mendelssohn's letter, promptly and effectively secured their stay.

In March 1771 Mendelssohn's health deteriorated so badly that Marcus Elieser Bloch, his doctor, decided his patient had to give up philosophy, at least temporarily. After a short and restless sleep one evening, Mendlessohn found himself incapable of moving and had the feeling of something lashing his neck with fiery rods, his heart was palpitating and he was in an extreme anxiety, yet fully conscious. This spell was then broken suddenly by some external stimulation. Attacks of this kind recurred. The cause of his disease was ascribed to the mental stress due to his theological controversy with Lavater. However, this sort of attack, in milder form, had presumably occurred many years earlier. Bloch diagnosed the disease as due to 'congestion of blood in the brain', and after some controversy this diagnosis was also accepted by the famous Hanoverian court physician, Johann Georg Ritter von Zimmermann, an admirer of Mendelssohn. In retrospect, his illness might be diagnosed as a heart-rhythm-problem and/or a mild form of familial dysautonomia, a hereditary disease of Ashkenazi Jews, which often brings with it a curvature of the spine and epilepsy like symptoms in times of stress.

Mendelssohn was treated with China bark, blood lettings on the foot, leeches applied to the ears, enemas, foot baths, lemonade and mainly vegetarian food. “No mental stress whatsoever” was ordered. However, although he remained subject to periods of setback, he eventually recovered sufficiently to write the major works of his later career.

It was after the breakdown of his health that Mendelssohn decided to "dedicate the remains of my strength for the benefit of my children or a goodly portion of my nation" – which he did by trying to bring the Jews closer to "culture, from which my nation, alas! is kept in such a distance, that one might well despair of ever overcoming it". One of the means of doing this was by "giving them a better translation of the holy books than they previously had". To this end Mendelssohn undertook his German translation of the Pentateuch and other parts of the Bible. This work was called the Bi'ur (the explanation) (1783) and also contained a commentary, only that on Exodus having been written by Mendelssohn himself. The transliteration was in an elegant High German, designed to allow Jews to learn the language faster. Most of the German Jews in that period spoke Yiddish and many were literate in Hebrew (the original language of the scripture). The commentary was also thoroughly rabbinic, quoting mainly from medieval exegetes but also from Talmud-era midrashim. Mendelssohn is also believed to be behind the foundation of the first modern public school for Jewish boys, "Freyschule für Knaben", in Berlin in 1778 by one of his most ardent pupils, David Friedländer, where both religious and worldly subjects were taught.

Mendelssohn also tried to better the Jews' situation in general by furthering their rights and acceptance. He induced Christian Wilhelm von Dohm to publish in 1781 his work, On the Civil Amelioration of the Condition of the Jews, which played a significant part in the rise of tolerance. Mendelssohn himself published a German translation of the Vindiciae Judaeorum by Menasseh Ben Israel. The interest caused by these actions led Mendelssohn to publish his most important contribution to the problems connected with the position of Judaism in a Gentile world. This was Jerusalem (1783; Eng. trans. 1838 and 1852). It is a forcible plea for freedom of conscience, described by Kant as "an irrefutable book". Mendelssohn wrote:

Brothers, if you care for true piety, let us not feign agreement, where diversity is evidently the plan and purpose of Providence. None of us thinks and feels exactly like his fellow man: why do we wish to deceive each other with delusive words?

Its basic thrust is that the state has no right to interfere with the religion of its citizens, Jews included. While it proclaims the mandatory character of Jewish law for all Jews (including, based on Mendelssohn's understanding of the New Testament, those converted to Christianity), it does not grant the rabbinate the right to punish Jews for deviating from it. He maintained that Judaism was less a "divine need, than a revealed life". Jerusalem concludes with the cry "Love truth, love peace!" — in a quote from Zacharias 8:19.

Kant called this "the proclamation of a great reform, which, however, will be slow in manifestation and in progress, and which will affect not only your people but others as well." Mendelssohn asserted the pragmatic principle of the possible plurality of truths: that just as various nations need different constitutions – to one a monarchy, to another a republic, may be the most congenial to the national genius — so individuals may need different religions. The test of religion is its effect on conduct. This is the moral of Lessing's Nathan the Wise (Nathan der Weise), the hero of which is undoubtedly Mendelssohn, and in which the parable of the three rings is the epitome of the pragmatic position.

To Mendelssohn his theory represented a strengthening bond to Judaism. But in the first part of the 19th century, the criticism of Jewish dogmas and traditions was associated with a firm adhesion to the older Jewish mode of living. Reason was applied to beliefs, the historic consciousness to life. Modern reform in Judaism has parted to some extent from this conception. Mendelssohn grew ever more famous, and counted among his friends many of the great figures of his time. But his final years were overshadowed and saddened by the so called pantheism controversy. Ever since his friend Lessing had died, he had wanted to write an essay or a book about his character. When Friedrich Heinrich Jacobi, an acquaintance of both men, heard of Mendelssohn's project, he stated that he had confidential information about Lessing being a "spinozist", which, in these years, was regarded as being more or less synonymous with "atheist" - something which Lessing was accused of being anyway by religious circles. This led to an exchange of letters between Jacobi and Mendelssohn which showed they had hardly any common ground. Mendelssohn then published his Morgenstunden oder Vorlesungen über das Dasein Gottes (Morning hours or lectures about God's existence), seemingly a series of lectures to his oldest son, his son-in-law and a young friend, usually held "in the morning hours", in which he explained his personal philosophical world-view, his own understanding of Spinoza and Lessing's "purified" ("geläutert") pantheism. But almost simultaneously with the publication of this book in 1785, Jacobi published extracts of his and Mendelssohn's letters as Briefe über die Lehre Spinozas, stating publicly that Lessing was a self confessed "pantheist" in the sense of "atheist". Mendelssohn was thus drawn into a poisonous literary controversy, and found himself attacked from all sides, including former friends or acquaintances such as Johann Gottfried von Herder and Johann Georg Hamann. Mendelssohn wrote a reply addressed To Lessing's Friends (An die Freunde Lessings) and died on January 4, 1786 as the result of a cold contracted while carrying this manuscript to his publishers on New Year's Eve; Jacobi was held by some to have been responsible for his death.

Mendelssohn had six children, of whom only his second oldest daughter, Recha, and his eldest son, Joseph, retained the Jewish faith. His sons were: Joseph (founder of the Mendelssohn banking house, and a friend and benefactor of Alexander von Humboldt), Abraham (who married Lea Salomon and was the father of Fanny and Felix Mendelssohn); and Nathan (a mechanical engineer of considerable repute). His daughters were Dorothea, the mother of Philipp Veit (and subsequently the consort, and then wife, of Friedrich von Schlegel), Recha and Henriette, all gifted women. Recha's only grandson (son of Heinrich Beer, brother of the composer Giacomo Meyerbeer), was born and educated as a Jew, but died very young, together with his parents, apparently from an epidemic. Joseph Mendelssohn's son Alexander (d. 1871) was the last male descendant of Moses Mendelssohn to practice Judaism.