The Semag סמ״ג

Moses ben Jacob of Coucy was one of the Tosafists and one of those who defended the Talmud at the Paris disputation of 1240.

Before that, in 1235 in France and in 1236 in Spain, he travelled the communities preaching. He spoke of practical matters: of tefillin, of the mezuzah, of tzitzit — and of honesty in dealings with non-Jews, insisting that deceit there is graver, because it profanes the Name. In Spain he pressed for the dissolution of intermarriages.

Out of that journey came Sefer Mitzvot Gadol — a code in which the commandments are taken one by one, with their laws and their source. The book proved usable precisely because it was written by a man who knew what people ask: it answers questions rather than merely expounding.

The Semag became one of the principal sources for later codes, and Rabbi Joseph Karo's Beit Yosef cites it constantly.