Nissim of Gerona
Nissim ben Reuven (1320–9th of Shevat, 1376, Hebrew: נסים בן ראובן) of Girona, Catalonia was an influential talmudist and authority on Jewish law. He was one of the last of the great Spanish medieval talmudic scholars. He is also known as the RaN (ר"ן), the Hebrew acronym of his name, as well as the RaNbaR (רנב"ר), the Hebrew acronym of his full name, including his father's name, Reuven (ראובן). 
The RaN was born in Barcelona, Catalonia. He was a Physician and had knowledge of astronomy. The RaN's teacher is not known; his works refer to a R. Perez as "morenu" ("our teacher"), but it is thought that his father, Reuben b. Nissim, was his primary teacher. RaN was considered the greatest Halachic authority of his generation, and queries were sent to him from throughout the Jewish diaspora. His closest disciples were the Rivash (Isaac ben Sheshet) and Hasdai Crescas.
In his analyses, Nissim frequently refutes other rishonim; in his legal rulings however, he generally adopts their opinions in practice. He was "no friend of mysticism", and even reproved Nahmanides for devoting too much time to the Kabbalah.
His best-known work is his commentary and explanation of Alfasi's "Hilkhot", which had, at that time, been adopted for practical decisions. He is very detailed and explicit where the subject is important from a practical point of view, but extremely brief when dealing with theory. Ran did not write the commentary on all of Alfasi, although the whole commentary is often ascribed to him. Ran is now printed with Alfasi in all standard editions of the Talmud.
Ran also wrote a commentary on the Talmud. The commentary is oriented toward practical decisions as opposed to theory. The commentary has been printed only on some tractates, the rest exists in manuscript or has been lost.
Other works include, responsa, a recently published commentary on the Bible, a work of philosophy, and a collection of sermons ("Derashot Ha'Ran").
- Nimukei Yosef, Bava Metzia, 'ע"א, עמוד ב: on Gemara בבא מציעא עמוד קט"ז עמוד א
- Nissim B. Reuben Gerondi, jewishencyclopedia.com