In the Middle Ages, redeeming Jewish slaves gained importance and—up until the 19th century—Jewish congregations around the Mediterranean Sea formed societies dedicated to that purpose. Jewish communities customarily ransomed Jewish captives according to a Judaic mitzvah regarding the redemption of captives (Pidyon Shvuyim). In his "A History of the Jews", Paul Johnson wrote, "Jews were particularly valued as captives since it was believed, usually correctly, that even if they themselves poor, a Jewish community somewhere could be persuaded to ransom them. ... In Venice, the Jewish Levantine and Portuguese congregations set up a special organization for redeeming Jewish captives taken by Christians from Turkish ships, Jewish merchants paid a special tax on all goods to support it, which acted as a form of insurance since they were likely victims."