In 1763 the Berlin Academy set its prize question on whether metaphysics could reach the certainty of mathematics. It was won by a bookkeeper in a Berlin silk manufactory who had entered the city twenty years earlier, aged fourteen, through the Rosenthaler Gate — the one opening in the wall that Jews and cattle were permitted to use. Kant came second. That October the king granted the winner the standing of a protected Jew, securing his right to live in Berlin undisturbed, and the privilege covered him alone: not his wife, not his children.
The Enlightenment had made Europe's Jews an offer with an unwritten clause. Reason belongs to everyone and any mind may enter, but entry seemed to require arriving empty-handed, and what a Jew carried was a law, a calendar, a language, a community with courts of its own. Whether a Jew could accept the offer without surrendering all of that was, in the middle of the eighteenth century, an open question. Moses Mendelssohn was asked to answer it in public, by people who had already decided.
His father copied Torah scrolls. Mendelssohn taught himself Latin, bought Locke, the English philosopher, out of a bookkeeper's wages, and kept his post at the silk manufactory for the rest of his working life, because no university had a chair for him. The settled arrangement — a philosopher in German, a Jew at home — stopped being settled in October 1769. His dialogue on the immortality of the soul, Phädon, had made him a bestselling author across Protestant Germany two years earlier, and the Zurich pastor Johann Caspar Lavater used that fame to issue him a public challenge: refute the proofs of Christianity Lavater had just translated, or accept them. Mendelssohn answered that he could admire a thinker without taking his religion, and declined the contest; the refusal cost him anyway. In March 1771 his health broke down so badly that his physician ordered him off philosophy, at least for a time. The Academy elected him that same year, and Frederick the Great, Prussia's king, would not confirm it.
What he wrote afterwards had a different subject. At his urging, the Prussian official Christian Wilhelm von Dohm published in 1781 the first sustained argument that Jews should be admitted to civil rights. The next year Mendelssohn prefaced a German translation of a defence of the Jews by Manasseh ben Israel, the seventeenth-century Amsterdam rabbi, with a demand that the rabbis give up excommunication entirely — no religious body, his own included, should hold the power to punish. An anonymous pamphleteer saw the opening and took it: a Jew who denies that religion may coerce has knocked out the footing of his own law, which is coercion throughout, so he should be honest and convert.
The reply, in 1783, was Jerusalem, and its claim fits in a sentence: belief cannot be commanded, so nothing may command it. The state compels actions, because actions can be compelled; religion has only teaching and persuasion, and a conviction produced by force is not a conviction. Judaism, he argued, had never asked for one: what Sinai delivered was revealed legislation and not revealed doctrine — binding practice for those bound to it, containing no truth that unaided reason could not reach. A Jew could therefore be a citizen entire, with nothing held back and nothing put down at the door. Kant thought the argument could not be refuted.
He died on 4 January 1786, after catching a cold while delivering the manuscript of his last defence of his dead friend Lessing, the German dramatist and critic. Four of his six children left Judaism, and his grandchildren Felix and Fanny, both composers, were raised Christian — a fact used ever since both as the verdict on his project and as the measure of what emancipation actually cost. Historians still disagree over whether he fathered the Jewish Enlightenment or whether it simply gathered around him. The separation he argued for is now ordinary in liberal states.